WBti 






ill 



BE VISED EDITION. 



NATURAL PHILOSOPHY: 



EMBRACING 



THE MOST RECENT DISCOVERIES 



VARIOUS BRANCHES OF PHYSICS, 



AND EXHIBITING 



THE APPLICATION OF SCIENTIFIC PRINCIPLES IN EVEPvY-DAY LIFE. 



ADAPTED TO USE WITH OR WITHOUT APPARATUS, AND ACCOMPANIED WITS 

FULL DESCRIPTIONS OF EXPERIMENTS, PRACTICAL EXERCISES, 

AND NUMEROUS ILLUSTRATIONS. 



V 
BY G. P. QUACKENBOS, LL. D., 

PRINCIPAL OF "THE CQLLEGIATE SCHOOL *\ N. Y. : AUTHOR OF "FIRST LESSONS tM 

COMPOSITION "; " ADVANCED COURSE OF COMPOSITION AND RHETORIC ", 

"ILLUSTRATED SCHOOL HISTORY OF THE UNITED STATES'", ETC. 



NEW YORK •:• CINCINNATI .• CHICAGO 

A M E R I C A N B O OK CO M P A N Y. 



M 



Q^ 



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29661 



By the same Author: 

FIRST LESSONS IN COMPOSITION : In which the Principles of the Art. -are 
developed in connection with the Principles of Grammar. 12mo. pp. 182. 

ADVANCED COURSE OF COMPOSITION AND RHETORIC: A Series of 
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Language, Punctuation, Taste, the Pleasures of the Imagination. Figures, 
Style and its Essentia] Properties^ Criticism, and the various Departments 
of Composition : Illustrated with copious Exercises. 12mo, pp. 450. 

FIRST BOOK IN GRAMMAR: lGmo, pp. 120. 

AN ENGLISH GRAMMAR: 12mo, pp. 288. 

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A HIGHER ARITHMETIC. 



Copyright, 1871 and 1879, by G. P. Quackenbos, 
Copyright, 1887 and 1S99, by Louise B. Q,uack.knbos. 

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PREFACE 



The importance of the physical sciences is now so generally ad- 
mitted that there are few institutions of learning in which they are 
not made regular branches of study. And very properly. — for what 
can be more interesting and instructive, what more worthy of the 
attention of intelligent creatures, what more calculated to inspire 
their minds with a thirst for further knowledge, and fill their hearts 
with reverent gratitude to the Divine Being, than an acquaintance 
with the laws of the material world, the mysterious influences con- 
stantly at work in nature, and the principles by which atoms and 
worlds are alike controlled ? 

It is in the hope of investing this subject with a lively interest, 
and bringing it home to the student by exhibiting the application 
of scientific principles in every-day life, that the Natural Philosophy 
here presented to the public has been prepared. The author has 
sought to render a subject, abstruse in some of its connections, easy 
of comprehension, by treating it in a clear style, taking its princi- 
ples one at a time in their natural order, and illustrating them fully 
with the facts of our daily experience. The range of topics is com- 
prehensive. By avoiding unnecessary repetitions, room has been 
found for chapters on Astronomy and Meteorology; one of which 
subjects, at least, has heretofore been invariably omitted in similar 
treatises, though a summary of both is important, as time is seldom 
found for pursuing these branches in separate volumes. 

The incorrectness of many of the text-books on Natural Philos- 
ophy has been a subject of general complaint. Grave errors, both 
of theory and fact, have been handed down from one to another, and 
the results obtained by modern research have been too often over- 



4 .PREFACE. 

looked. In preparing this volume, every effort has been made to 
ensure accuracy, the most recent authorities have been consulted, 
and it is believed that a faithful view is presented of the various 
sciences embraced, as far as they are at present developed. It is 
Lhe intention of the author to keep his book up to the times by 
constant revision, and to make such alterations and additions as the 
progress of discovery may require. 

Two styles of type are used in the text ; a larger size for lead- 
ing principles, a smaller size for descriptions of apparatus and exper- 
iments, explanatory illustrations, &c. By confining a class to the 
former when the saving of time is an object, a brief yet complete 
course may be taken. Questions at the bottom of each page will 
be found to facilitate the examiner's duty, and to afford the pupil a 
means of testing his preparation before reciting. At the end of 
such chapters as admit of it, easy practical examples have been in- 
troduced, to illustrate the rules and principles set forth. 

An important feature of this work is its adaptation to use with 
or without apparatus. The majority of schools have few facilities 
for experimental illustration. The wants of these are here met by 
a free use of engravings, full descriptions of experiments, and expla- 
nations of their results. 
New Yobk., July 1st, 1859. 



Kecent discoveries in the different departments of Physics, and 
the general acceptance of new theories with respect to the kindred 
forces, Heat, Light, and Electricity, having rendered necessary 
certain alterations in the text, the opportunity has been improved 
to revise the whole work; and this New Edition is offered to the 
public in the belief that it will be found in all respects accurately 
to represent the present state of science. 

New Yobk, January 2d, 1871. 



CONTENTS 



CHAPTER PAGK 

I. — Matter and its Forms .... ... 7 

II. — Properties of Matter 12 

III. — Mechanics. 

Motion. — Momentum. — Striking Force 26 

IV. — Mechanics {continued). 

Laws of Motion 34 

V. — Mechanics {continued). 

Gravity 46 

VI. — Mechanics {continued). 

Centre of Gravity . . . 70 

VII. — Mechanics {continued). 

The Motive Power. — The Resistance. — The Machine. — 

Strength of Materials 81 

VIII. — Mechanics (continued). 

The Mechanical Powers 94 

IX. — Mechanics {continued). 

Wheelwork. — Clock and Watchwork 120 

X. — Mechanics {continued). 

Hydrostatics 130 

XI. — Mechanics {continued). 

Hydraulics 152 

XII. — Pneumatics 165 

XIII. — Ptronomics 192 

The Steam Engine 219 



CONTENTS. 



CHAPTER 

XIV— Optics 
XV. — Acoustics . 
XVI. — Electricity 

Fr ictional Electricity 
Voltaic Electricity 
Thermo-electricity 
XVII. — Magnetism . 

Electro-magnetism 
Magneto-electricity 
XVIII. — Astronomy 

XIX. — Meteorology 

Figures reproduced 
Index . 
Appendix. 

The Phonograph . 
The Telephone . 
Edison's Electric Light 



PAGE 

229 

274 



290 
316 



349 
366 
368 
401 
407 
440 

451 

453 
455 



NATURAL PHILOSOPHY. 



CHAPTER I. 

MATTER AND ITS FORMS. 

1. Matter. — Whatever occupies space, whatever we can 
perceive by the senses, is known as Matter. 

Different kinds of matter are called Substances. Earth, 
water, air, are Substances, being different kinds of matter. 

A distinct portion of matter is called a Body. The 
earth, a rain-drop, a bubble, are Bodies. 

2. Force. — Whatever acts on a body, to change its form, 
state, or relation to other bodies, is a Force. Heat, light, 
electricity, are Forces. It is by the action of Forces on 
matter that the various phenomena of the universe are 
produced. 

3. Forms of Matter. — Matter exists in three forms ; 
Solid, Liquid, and A-er'-i-form. 

A body is said to be Solid, when its particles, while 
they possess the power of vibration within certain limits, 
yet cohere so strongly as to oppose a decided resistance to 
impression or penetration by other bodies ; example, ice. 
Solid bodies are called Solids. 

1. What is Matter? What are different kinds of matter called ? Give examples. 
What is a Body ? Give examples. 2. What is a Force ? What are produced by 
the action of forces on bodies? 3. In how many forms does matter exist? "Name 



8 MATTER AND ITS FORMS. 

A body is said to be Liquid, when its particles cohere 
so slightly that they can not only vibrate to and fro, but 
also roll freely around each other, and thus oppose but 
little resistance to penetration by other bodies ; example, 
water. Liquid bodies are called Liquids. 

Aeriform means having the form of air, and matter is 
said to exist in this state when its particles repel each other, 
tending to separate and spread out indefinitely ; example, 
steam. Aeriform bodies are called Gases and Vapors. 

Liquid and aeriform bodies are embraced under the 
general name of Fluids. 

There are marked points of difference between solids and fluids. A solid 
has a permanent shape ; a fluid accommodates its shape to that which con- 
tains it. A solid may often be moved by moving a portion of its particles ; 
as a pitcher by its handle. Not so with a liquid ; when we move only a 
portion of its particles, the rest are detached by their own weight. Thus, 
by dipping a tumbler into a pail of water, we can not remove all the fluid, 
but only as much as the tumbler contains. 

The same substance may, under different circumstances, appear in all 
three of these forms. Thus, water is a liquid ; when frozen, it becomes ice, 
which is a solid ; when exposed to a certain degree of heat, it is converted 
into steam, which is aeriform. 

4. Classes of Bodies. — Bodies are distinguished as 
Simple and Compound. 

A Simple Body consists of matter that can not be re- 
solved into more than one element ; as, gold. 

A Compound Body consists of matter that can be re- 
solved into two or more elements ; as air, which is com- 
posed of two gases. 

The simple bodies, or elements, of which every thing in the universe 
is composed, are sixty-four in number. Of these, fifty-one, distinguished 
by a peculiar lustre, are called Metals. The remaining thirteen are known 
as Non-metallic Elements. 

them. When is a body said to be solid ? What are solid bodies called ? When is 
a body said to be liquid? What are liquid bodies called? What does aeriform 
mean? When is a body said to be aeriform? What are aeriform bodies called? 
What name is applied to both liquid and aeriform bodies? Mention some of the 
marked points of difference between solids and fluids. In how many forms may the 
same substance appear? Give an example. 4. Into how many classes are bodies di- 
vided? Name them. What is a Simple body ? What is a Compound body? How 
many simple bodies are there? How are they divided ? Name the principal met- 



SIMPLE SUBSTANCES. 9 

The principal metals are the seven known to the ancients,— gold, silver, 
iron, copper, mercury, lead, and tin ; antimony, which was next discovered, 
in 1490 ; bismuth, zinc, arsenic, cobalt, plat'-i-num, nickel, manganese, &c. 
The thirteen non-metallic elements are ox'-y-gen, hy'-dro-gen, ni'-tro-gen, 
chlorine [klo'-reen], iodine [i'-o-deeri], bromine [bro'-meen], fluorine [flu'-o- 
reeri], se-le'-ni-um, sulphur, phosphorus, carbon, sil'-i-con, and bo'-ron. 

These simple substances are rarely found ; nearly every body that we 
meet with is composed of two or more elements, and is therefore compound. 
Such is the case with air, which was anciently thought to be a simple sub- 
stance, but was proved, towards the close of the eighteenth century, to be 
a mixture, by weight, of 23 parts of oxygen and 76 parts of nitrogen. Water, 
also, has been found to be a compound substance, made up of oxygen and 
hydrogen combined in the proportion, by weight, of 8 to 1. Of the sixty- 
four elements referred to above, twenty are so rare that their properties are 
not fully known ; thirty more are comparatively seldom met with ; the 
remainder constitute the great bulk of the globe and all that is thereon. 

The consideration of the simple substances, with their 
properties and combinations, belongs to the science of 
Chemistry. The force that causes them to combine and 
produce compound substances, is called Chemical Affinity. 
Oxygen and hydrogen combine and form water, in conse- 
quence of their chemical affinity. 

Chemical affinity subsists only between certain substances. If sulphuric 
acid be poured on a piece of zinc, the two substances will combine and 
form a compound entirely different from either. Pour the acid on a lump of 
gold, and no such change will ensue, because there is no chemical affinity be- 
tween them. 

5. Natural Philosophy. — Natural Philosophy is the 
science that treats of the properties and laws of matter. It 
is also called Physics. 

Pythagoras was the first to use the term philosophy. From him and his 
followers it was borrowed by Socrates ; who, w r hen the other sages of his time 
called themselves sophists, or wise men, modestly declared himself a philoso- 
pher, or lover of wisdom. — Philosophy implies a search for truth ; and Natu- 
ral Philosophy, as distinguished from Moral and Intellectual Philosophy, 
searches for the truths connected with the material world. 



als. Name the thirteen non-metallic elements. What is said of the simple substances ? 
What kind of substances are air and water? Of what is air composed? Of what, 
water ? flow many elements constitute the great bulk of the globe ? What is said 
of the rest? To what science does the consideration of the simple substances be- 
long? What causes the simple substances to combine? Give an instance of chem- 
ical affinity. Illustrate the fact that chemical affinity subsists only between certain 
substances. 5. What is Natural Philosophy ? With whom did the term philosophy 
1- 



10 MATTER AND ITS FORMS. 

6. Modes of Investigation. — We arrive at the facts of 
Natural Philosophy in two ways ; by Observation and Ex- 
periment. Observation consists in watching such phenom- 
ena, or appearances, as occur in the course of nature. 
Experiment consists in causing such phenomena to occur 
when and where we wish, for the purpose of noting the 
attendant circumstances and results. 

For example, we arrive at the fact that an unsupported body will descend 
to the earth's surface, when we see an apple fall from a bough ; this is by Ob- 
servation. We learn the same fact, when, with the view of ascertaining 
what it will do, we let an apple drop from our hands ; this is by Experiment. 

7. Modes of Measoni?ig. — Having obtained our facts in 
the two ways just described, and classified them, we next 
proceed from individual cases to deduce general laws. This 
is called Reasoning by Induction. 

Thus, if we try the experiment "with many different apples, and find that 
each, when let go, will fall to the ground, we lay down the general law that 
all apples will fall in like manner. If we find that not only apples do this, 
but also other objects with which we make the trial, we go a step furthe:-, 
and announce another law, that all objects left unsupported will fall to the 
ground. 

It is by this process that most of the laws and principles of Natural Phi- 
losophy have been established. Archimedes [ar-ke-me'-deez], the Sicilian 
philosopher, used it over two thousand years ago. Gal-i-le'-o revived it in 
modern times v and it may be said to lie at the foundation of all the great dis- 
coveries of Newton. 

When we have two similar phenomena and know that 
one proceeds from a certain cause, we attribute the other 
to the same cause. This is called Reasoning by Analogy. 

Such reasoning is employed in the case of all bodies that are beyond our 
reach. From what is near, we draw conclusions respecting what is remote. 
It is thus, for example, that the philosopher explains the motions of the heav- 
enly bodies, extending to them, by analogous reasoning, the same principles 
that govern the motion of bodies on the earth. 

8. Division of the Subject. — Natural Philosophy, hav- 



•riginate? Who borrowed it from Pythagoras? What does philosophy imply? 
What is the particular province of Natural Philosophy ? 6. How do we arrive at 
the facts of Natural Philosophy? In what does Observation consist? In what, Ex- 
periment? Illustrate these definitions. 7. What is meant by reasoning by induc- 
tion? Give an example. By what philosophers has this mode of reasoning been 
employed? What is meant by reasoning by analogy f Give an example. 8. What 



DIVISION OF THE SUBJECT. 11 

ing to treat of matter in all its forms, as well as the forces 
that act upon it, embraces the following branches : — 

Mechanics, which treats of forces and their application 
in machines. To Mechanics belong 

Hy-dro-stat'-ics, which treats of liquids at rest ; 

Hy-drau'-lics, which treats of liquids in motion. 

Pneumatics [nu-mat'-ics], which treats of gases and 
vapors. 

Pyr-o-nom'-ics, which treats of heat. 

Optics, which treats of light and vision. 

Acoustics [a-cow 1 'sties'], which treats of sound. 

Electricity, which treats of the force so called. To 
Electricity belong 

Galvanism, which treats of electricity produced by 
chemical action ; 

Thermo-electricity, which treats of electricity developed 
by heat ; 

Magneto-electricity, which treats of electricity devel- 
oped by magnetism. 

Magnetism, which treats of magnets and the forces they 
develop. To Magnetism belongs 

Electro-magnetism, which treats of magnetism devel- 
oped by electricity. 

Astronomy, which treats of the heavenly bodies. 

Me-te-o-rol'-o-gy, which treats of the phenomena of the 
atmosphere. 



branches does Natural Philosophy embrace ? Of what does Mechanics treat ? Hy- 
drostatics? Hydraulics? Pneumatics? Pyronomics? Optics? Acoustics? Elec- 
tricity ? Galvanism ? Thermo-electricity ? Magneto-electricity ? Magnetism ? 
Electro-magnetism? Astronomy? Meteorology? 



12 PBOP-tiRTIES OF MAlTJflB. 



CHAPTER II. 

PROPERTIES OF MATTER. 

9. Every distinct portion of matter possesses certain 
properties. Some of these belong in common to all bodies, 
solid, liquid, and aeriform, and are called Universal Prop- 
erties of matter. Others, again, are found only in certain 
substances, and these are known as Accessory Properties. 

The Universal Properties of matter are Extension, Fig- 
ure, Impenetrability, Indestructibility, Inertia [in-er f -sha], 
Divisibility, Porosity, Compressibility, Expansibility, Mo- 
bility, and Gravitation. 

The principal Accessory Properties are Cohesion, Ad- 
hesion, Hardness, Tenacity, Elasticity, Brittleness, Mallea- 
bility, and Ductility. 

We proceed to consider these properties in turn. 

10. Extension. — Extension is that property by which 
a body occupies a certain portion of space. The portion 
of space thus occupied is called its Place. 

In other words, every body, however small, must have some size, or a 
certain length, breadth, and thickness, which are called its Dimensions. 
The greatest of these three dimensions is its Length ; the next greatest, its 
Breadth, or Width ; the least, its Thickness. But, instead of any of these 
terms, we use the word height to denote distance from bottom to top in the 
case of objects towering above us, and depth to denote distance from top to 
bottom in the case of objects extending below us. 

11. Figure. — Figure is that property by which a body 
has a certain shape. 

This property necessarily follows from Extension ; for since every body 
must have length, breadth, and thickness, it must also have some definite 

9. What is meant by Universal Properties of matter ? What is meant by Acces- 
sory Properties ? Enumerate the universal properties. Mention the principal ac- 
cessory properties. 10. What is Extension ? What is meant by the dimensions of a 
body ? What is Length ? Breadth ? Thickness ? When are the terms height and 
depth used ? 11. What is Figure ? From what does figure follow ? What is the 



IMPENETRABILITY. 13 

shape. While this is true of all bodies, it must be remembered that the form 
of solids is permanent, while that of fluids varies, to adapt itself to every new 
surface with which it comes in contact. A bullet keeps the same shape, 
w::erever it is placed ; whereas a quantity of water, poured from a tumbler 
into a pail, visibly changes its form. 

12. Impenetrability. — Impenetrability is that property 
by which a body occupies a certain portion of space, to the 
Bxclusion for the time of all other bodies. 

Impenetrability may be illustrated with a variety of simple experiments. 
Fill a tumbler to the brim with water, and drop in a bullet ; the water will at 
once overflow Fill a bottle with water, and try to put the cork in ; the cork 
will not enter till it has displaced some of the water : if it fit so closely that 
tho water can not escape, and a hard pressure be exerted, the bottle will 
burst. 

The impenetrability of air is shown with the ap- 
paratus represented in Figure 1. A is a glass jar 
fitted with an air-tight cork, through which a funnel, 
B, enters the jar. C is a bent tube, one end of which 
also passes through the cork into the jar, while the 
other is received in a glass of water, D. Let water 
be poured into the funnel ; as it descends, drop by 
drop, into the jar, air passes out through the bent 
tube, and escapes through the water in D in the form 
of bubbles. Thus it is shown that water and air can 
not occupy the same space at the same time. 

13. Impenetrability belongs to all substances, though in some cases it 
may appear to be wanting. A nail, for instance, is driven into a piece of 
wood without increasing its size ; but it effects an entrance by forcing to- 
gether the fibres of the wood, not by occupying their space at the same time 
with them. In like manner, a certain amount of salt and sugar may be suc- 
cessively dropped into a tumbler brim-full of water without causing it to over- 
flow. The particles of water, which are supposed to be globular, do not 
everywhere touch each other, and the particles of salt are accommodated in 
the interstices between them. These in turn leave minute Fig. % 
spaces, into which the still smaller particles of sugar find their ^p Z-— y^IN 
way. Fig. 2 exhibits such an arrangement. To illustrate it r^i^J^,^ 
familiarly, we may fill a vessel with as many orange^ s it f ( ; j 
will hold, and then pour on a quantity of peas, shaking the 9^7^ 
vessel slightly so that they may settle in the empty spaces. 



difference between solids and fluids as regards figure ? 12. "What is Impenetrability? 
Give some familiar illustrations of this property. Describe the experiment with the 
apparatus represented in. Fig. 1. 13. "What is said of those cases in which impen- 
etrability appears to be wanting ? Illustrate this with the nail. Explain how salt 
and sugar may be dropped into a tumbler full of water without causing it to Qve?- 





14 PROPERTIES OF MATTER. 

When the vessel will receive no more peas, repeat the process with fine grav- 
el, and it will be found that a considerable quantity will lodge between the 
oranges and peas. 

14. Indestructibility. — Indestructibility is that prop- 
erty which renders a body incapable of being destroyed. 

Matter may be made to assume a new form and new 
properties, but it can not cease to exist. The quantity of 
matter now in the world is precisely the same as when it 
was first called into being, and it will continue undimin- 
ished till the end of time. The Deity alone created, and it 
is only He that can destroy. 

15. To this universal law we have some apparent exceptions; but, when 
closely examined, it will be found that they are exceptions in appearance 
only. Water, for instance, exposed to the air in a shallow dish, will at length 
disappear by evaporation ; but it is not destroyed. Assuming the form of 
vapor, it ascends, becomes incorporated with clouds, is condensed into rain, 
and falls, — to go through the same process again. — The oil in a burning lamp 
gradually gets lower and lower till at last it is all gone, and we say it is 
burned up ; but the process of combustion, or burning, only changes it into 
invisible gases, — not one particle of its substance is lost. In like manner, 
when fuel of any kind is consumed, there is only a change of form, not a de- 
struction of the least portion of matter. 

Such changes are constantly going on in the operations of nature. One 
body perishes, and of the materials that composed it another is formed. Our 
own frames may contain particles that were in the bodies of Adam, Noah, or 
Socrates ; or, if they do not now, may do so to-morrow, for they are constant- 
ly parting with portions of their substance, the place of which is as con- 
stantly supplied by new matter. It is supposed that the whole body, in- 
cluding even the innermost parts of its hardest bones, is completely renewed 
every seven years. Yet, amid all the countless transitions of nature, not a 
single particle of matter is destroyed or lost. 

16. It was by a knowledge of the indestructibility of matter that Sir Wal- 
ter Raleigh is said to have won a wager of Queen Elizabeth. Having weighed 
out a sufficient quantity of tobacco to fill his pipe, he came into the queen's 
presence, and as the wreaths of smoke curled up offered to bet her Majesty 
that he could tell their weight. Elizabeth accepted the bet, and Sir Walter 
quietly finished his pipe ; then, having shaken out the ashes, he weighed 
them, and, subtracting the amount from that of the tobacco originally put 



flow. 14. What is Indestructibility ? What can be dene to matter, and what not ? 
35. What is said of the apparent exceptions to this law ? What becomes of water 
exposed to the air ? What becomes of the oil in a burLing lamp ? What is .said of 
the changes of nature ? What is said of the changes in the human body ? 16. How 
did Sir Walter Raleigh teach Queen Elizabeth that matter is indestructible * 17. What 



INEKTEA. 15 

In, told the queen the exact weight of the smoke. Elizabeth paid the wager, 
and thus learned to her cost that matter is indestructible. 

1 7. Inertia. — Inertia is that property which renders a 
body incapable of putting itself in motion when at rest, or 
coming to rest when in motion. 

When a stationary body begins to move, or a moving 
body comes to rest, it is not through any power of its own, 
but because it is acted on by some external agency, which 
we call a Force. 

That no inanimate body can put itself in motion, is evident from our daily 
experience. The rocks that we saw on the earth's surface ten years ago are 
to-day in precisely the same place as the)" then were, and there they will re- 
main forever unless some force removes them. 

It is equally true, though not so obvious, that a body once in motion can 
not of itself cease to move. The earth revolves on its axis, the heavenly 
bodies move in their orbits, just as they did at the time of the Creation; they 
have no power to stop. It is true that on the surface of the earth a moving 
body gradually comes to rest, when the force which put it in motion ceases 
to act ; but this is owing to the resistance of the air and a force which draws 
it towards the centre of the earth — not to any agency of its own. Remove 
all external forces, and its inertia would keep it moving on in a straight line 
forever. 

18. Familiar Examples. — It is in consequence of inertia that a horse has to 
strain hard at first to move a load, which, when it is once in motion, he can 
draw with ease. A car, through its inertia, continues moving after the loco- 
motive is detached. Through inertia, a person standing erect in a stationary 
boat or wagon is thrown backward if it suddenly starts : his feet, touching 
the bottom, are carried forward with it, while his body by its inertia does not 
partake of the onward motion and falls backward. So, a person standing 
erect in a boat or wagon that is moving rap- „. „ 

idly, is thrown forward if it suddenly stops ; 
his feet cease to move at once, while his body 
continues in motion in consequence of its iner- 
tia, and falls forward. 

19. An interesting experiment to illustrate 
inertia may be performed with the apparatus 
represented in Fig. 3. On the top of a short 
pillar is placed a card, and on the card a brass 
ball. Beside the pillar is fixed a steel spring, 
with an apparatus for drawing it back. If the 

is Inertia? What is a Force ? What evidences of the inertia of matter have we in 
nature ? If inertia is one of the properties of matter, why does a moving body come 
to rest on the earth's surface ? 18. Give some familiar examples of inertia and its 
;onsequences. 19. Describe the experiment with the inertia apparatus. "Describe 




16 



PROPERTIES OF MATTER. 



spring is drawn back and then suddenly released, it will drive the card from 
the top of the pillar, while the ball in consequence of its inertia will retain 
its place. 

Those who have not the above apparatus may balance a card with a penny 
placed upon it on the tip of one of the fingers of the left hand, and strike it 

suddenly with the middle finger 
of the right hand, as represented 
in Fig. 4. If properly balanced 
and evenly struck, the card will 
/l^^^^^^^lj r 1 **' fly away, and the penny will be 

] f ^^ " ( \v^- % left on the finger. 

^R\ \sf) ^*v^ jfiltK. I n these cases, there is not 

sufficient time for the card to 
overcome the inertia of the ball 
and the penny, and impart to 
them its own motion. When, 
however, motion has once been communicated by one body to another rest- 
ing on it, the inertia of the latter keeps it in motion. A person riding in a 
carriage partakes of its motion, and if he jumps from it runs the risk of being 
thrown down, because his feet cease to move the instant they strike the 
ground, while the inertia of his body carries it forward. The circus-rider 





Fig. 5. 



v^tjnfp^ 



takes advantage of this fact. 
While his horse is going at 
full speed, he jumps over a 
rope extended across the 
ring (see Fig. 5), and re- 
gains his footing on the 
saddle without difficulty. 
To do this, he has only to 
leap straight up as he comes 
to the rope, for his inertia 
bears him along in the same 
direction as his horse. 
A bullet thrown at a pane of glass breaks it into many pieces, but, fired 
at it from a rifle, merely makes a circular hole. In the latter case, all the par- 
ticles of glass, on account of their inertia, can not immediately acquire the 
rapid motion of the bullet; and consequently only that portion which is 
struck is carried onward. On the same principle, a thin stick resting on twc 
wine-glasses (see Fig. 6) may be broken by a quick blow with a poker in its 
centre, without injury to its brittle supports. 




the experiment with the card and penny. What is the effect of inertia, when motion 
has once been communicated to a body? Why is a person who jumps from a car- 
riage in motion thrown down ? Explain the leap of the circus-rider. What is the 
effect of throwing a bullet against a pane of glass, and what of firing it ? What causes 
the difference ? What experiment may be performed to illustrate this point ? 20. To 



DIVISIBILITY. 



17 




Fig. 7. 



20. The heavier a body is, 
the greater is its inertia ; the 
more strongly does it resist 
forces that would set it in 
motion, change its motion, or 
stop its motion. 

Instinct teaches this fact. A child, 
when nearly overtaken by a man, will 
suddenly turn, or " dodge" as he calls 
it, thus gaining ground, inasmuch as 
the greater weight and inertia of the man compel him to make a longer turn. 
So a hare, in making for a cover, often escapes a hound by making a num 
ber of quick turns. The greater inertia of the 
hound carries him too far, and thus obliges 
him to pass over a greater space, as seen in 
Fig. 7, in which the continuous line shows the 
hare's path and the dotted line the hound's. 

2 1 . Divisibility. — Divisibility 
is that property which renders a 
body capable of being divided. 

Atomic Theory. — Practically, 
there is no limit to the divisibility 
of matter. Most philosophers, how- 
ever, hold what is called the Atom- 
ic Theory, — that if we had more 
acute senses and instruments sufficiently delicate, we would 
at last, in dividing and subdividing matter, arrive at ex- 
ceedingly small particles, incapable of further division. 
Such particles they call Atoms, a term derived from a 
Greek word meaning indivisible. 

According to this theory, <lifferent kinds of matter are 
made up of different kinds of atoms ; but in the same sub- 
stance the atoms are always the same in shape and nature. 
It must be remembered, however, that no particle has yet 
been arrived at that can not be divided. 

22. Instances of Divisibility. — Matter has been divided into parts incredi- 




what is a body's inertia proportioned ? How do children turn this fact to account ? 
How does the hare apply this principle? 21. What is Divisibility? Is there any 
limit to the divisibilitv of matter ? Give the chief points of th* Atomic Theory 

2 



18 PROPERTIES OP MATTER. 

bly minute. With the proper instrument, ten thousand distinct parallel lines 
can be drawn on a smooth surface an inch in width. So minute are these 
lines that they can not be seen without a microscope, not even a scratch be- 
ing visible to the naked eye. 

A grain of musk will diffuse a perceptible odor through an apartment for 
twenty years. It does this by filling the air with particles of its substance ; 
but so inconceivably minute are these particles, that, if the musk is weighed 
at the end of the twenty years, no loss of weight can be detected. 

A grain of copper dissolved in nitric acid will impart a blue color to three 
pints of water. Each separable particle of water must contain a portion of 
the grain of copper, — which is thus, it has been computed, divided into no 
less than 100,000,000 parts. 

23. Nature affords many striking examples of the divisibility of matter. 
The spider's web is so attenuated that a sufficient quantity of it to go around 
the earth would weigh only eight ounces ; and yet this minute thread con- 
sists of about a thousand separate filaments. 

Blood is composed of small red globules floating in a colorless liquid. Of 
these globules, every drop of human blood contains at least a million. Mi- 
nute as they are, they may be divided into globules much more minute. As 
we descend in the scale of creation, we come to animals whose whole bodies 
are no larger than these little globules of human blood, yet possess all the 
organs necessary to life. How inconceivably small are the vessels through 
which the fluids of their bodies must circulate ! 

The microscope reveals to us wonders of animal life that are almost in- 
credible. It shows us in duck-weed animalcules so small that it would Lake 
ten thousand millions of them to equai the size of a hemp-seed. In a single 
drop of ditch-water, it exhibits myriads of moving creatures. The mineral 
called tripoli is formed of these animalcules fossilized or turned into stone ; 
and it has been shown that the thirtieth part of a cubic inch of this sub- 
stance contains the bodies of more than thirteen hundred million animal- 
cules — nearly as many as the whole number of human beings on the globe. 

24. Porosity. — The shape of the atoms of different 
bodies, we have no means of determining. By reason of 
their shape, however, or from some other cause, they do 
not everywhere touch each other, but are separated by 
interstices, to which we give the name of Pores. Pores 
are often visible to the naked eye, as in sponge and pumice- 
stone ; in other cases, as in gold and granite, they are too 
minute to be detected even with the microscope. 

22. How has the divisibility of matter been illustrated with a smooth surface an inch 
in width ? How does a grain of musk prove divisibility ? How, a grain of copper? 
'23. What is said of the spider's web ? Mention some examples of the divisibility of 
matter afforded by nature. What does the microscope reveal to us ? Mention some 
of these wonders. 24. What are Pores ? What is said of the difference in the size 



POROSITY. 1 9 

25. Porosity is the property of having pores. It be- 
longs to all bodies. 

26. That water is porous, is proved by the fact that a vessel filled with it 
will receive considerable quantities of salt and sugar without overflowing. 
What can become of these substances, unless, as shown in Fig. 2, their par- 
ticles lodge in the interstices between the particles of water ? It is on this 
principle that hot water receives more salt and sugar without overflowing 
than cold. Heat expands water, — that is, forces its particles further apart, — 
and thus enables a greater quantity of salt and sugar to lodge between them, 

That granite is porous, is shown by placing a piece of it in a vessel oi 
water under the receiver of an air-pump (described on page 178), and remov- 
ing the air. Little bubbles will soon be seen rising through the water. These 
bubbles are the air contained in the invisible pores of the granite. 

A piece of iron is made smaller by hammering. This proves its porosity. 
Its particles could not be brought into closer contact, if there were no inter- 
stices between them. 

27. An experiment performed some years ago at Florence, Italy, to ascer- 
tain whether water could be compressed, proved that gold is porous. A vio- 
lent pressure was brought to bear on a hollow sphere of gold filled with water. 
The water made its way through the gold and appeared on the outside of the 
sphere. Water will thus pass through pores not more than one half of the 
millionth of an inch in diameter. 

28. Density and Rarity. — The fewer and smaller the 
pores in a body, the more compact are its particles, and 
the greater is the weight of a given bulk. Bodies whose 
particles are close together are called Dense; those with 
large or numerous pores are called Rare. 

29. Compressibility and Expansibility. — These two 
properties are the opposites of each other. Compressibility 
is that property which renders a body capable of being re- 
duced in size. Expansibility is that property which renders 
a body capable of being increased in size. 

Compressibility and Expansibility follow from porosity. 
Since the particles of bodies do not everywhere touch each 
other, the application of a sufficient force will bring them 
closer together, and the size of the bodies will thus be re- 

of the pores ? 25. What is Porosity ? 26. How is water proved to be porous ? Why 
does hot water receive more salt and sugar than cold ? How may it be proved that 
granite is porous ? How is the porosity of iron proved ? 27. Give an account of the 
experiment by which the porosity of gold was proved. How small pores will water 
pass through ? 28. What bodies are called dense ? What bodies are called rare t 
29. What is Compressibility ? What is Expansibility ? Show how these properties 



20 



PROPERTIES OF MATTER. 



Fig. 8. 




duced. A sponge, for instance, by the simple pressure of the 
hand, can be reduced to one-tenth of its natural size. In like 
manner, if the pores of a body are made larger by any agency 
(as they are by heat), its size is proportionately increased. 
30. All bodies possess these properties. A rod of iron, 
too large to enter a certain opening, may be so compressed by 
hammering as to pass through it, and then so expanded by 
heat as to render its entrance again impossible. Liquids, 
which were long considered incompressible, are now known 
to yield to a high degree of pressure ; their expansibility is 
illustrated by the rise of mercury in the thermometer. 

The compressibility and expansibility of air are shown 
by the apparatus represented in Fig. 8. Let P be a piston, 
fitted, air-tight, to the cylinder A B. As the piston is driven 
down, the air, unable to escape, is compressed; as it is 
drawn back, the air expands. 

Aeriform bodies are more easily compressed and ex- 
panded than any others. 

31. Mobility. — Mobility is that property which renders 
a body capable of being moved. 

Though the inertia of bodies prevents them from mov- 
ing themselves, yet there is no body that can not be moved 
by the application of a proper force. 

32. Gravitation. — Gravitation, or the Attraction of 
Gravitation, is a force residing in every body, by virtue 

Fig. 9. of which it tends to draw every other body to 
itself. A cannon-ball dropped from the hand 
falls to the earth by reason of the attraction 
of gravitation. The earth at the same time 
moves towards the cannon-ball, but through 
a space inconceivably small in consequence ot 
its vast superiority in size over the ball. 

That the cannon-ball is capable of attracting as well as 
being attracted, may be proved by suspending two balls close 
to each other by very long cords. In consequence of the 
attraction of the halls, the cords will not hang parallel, but 
will incline towards each other as they descend, as shown 
in Fig. 9. 



follow from porosity. 30. How may compressibility and expansibility be illustrated 
with an iron rod ? What is said of these properties in liquids? How may the com- 
pressibility and expansibility of air be shown ? What bodies are most easily com- 



COHESION. 



21 



We now proceed to the Accessory Properties, which 
are confined to certain bodies. 

33. Cohesion. — Cohesion is that property by which the 
particles of a body cling to each other. As particles are 
also called mol'-e-cules, Cohesion has received from some 
authors the name of Mo-lec'-u-lar Attraction. 

Cohesion belongs particularly to solids, and is in fact the cause of their 
solidity. In some it is stronger than in others, rendering them harder or 
iiore tenacious. Liquids have so little cohesion that their weight alone over- 
comes it, and causes a separation of particles. In aeriform fluids cohesion 
is entirely wanting, its place being supplied by a Repulsive Force, which 
tends to make their particles spread out from each other. 

34. Adhesion. — Adhesion is that property by which the 
surfaces of two different bodies placed in contact cling to- 
gether. 



The bodies in question may 
be of the same kind of mat- 
ter. This is proved by an ex- 
periment with two glass plates 
ground perfectly even. Let 
these be pressed together, and 
it will be found, on attempt- 
ing to pull them apart by their 
handles, that considerable 
force will be required. The 
larger the surfaces of the 
plates, the harder it will be to 
separate them. A pair of Ad- 
hesion Plates is represented 
in Fig. 10. 

Adhesion also operates 
between the surfaces of sol- 
ids and liquids. Suspend a 
piece of copper-plate from 
one side of a pair of scales, 
in such a way that its under 
surface may be parallel to 
the floor, and balance it with 
weights placed in the scale 
on the other side. Then, 
without disturbing the cop- 



Jig. 10. 




ADHESION PLATES. 



Fig. 11. 



t 




pressed and expanded? 31. What is Mobility? 32. What is Gravitation? How 
!oes it operate in the case of a cannon ball dropped from the hand to the earth ? 
How does it operate in the case of two cannon-balls suspended close to each other ? 



22 PROPERTIES OF MATTER. 

per, place a vessel beneath it, as in Fig. 11, and pour in water till the liquid 
just reaches the plate. The adhesion between the solid and the liquid is now 
so strong that additional weights (more or less, according to the extent of 
surface) may be put in the scale on the other side without causing them to 
separate. 

35. Hardness. — Hardness is that property by which a 
body resists any foreign substance that attempts to force a 
passage between its particles. 

The hardness of a body depends on the degree of firm 
ness with which its particles cohere. It is therefore en- 
tirely distinct from density, which depends on the number 
of particles in a given bulk. Thus lead is dense, but not 
hard. 

Neither liquids nor aeriform fluids possess this property ; and even in 
some solids, for instance butter and wax, it is almost entirely wanting. 

Of two bodies, that is the harder which will scratch the surface of the 
other. By trying the experiment with different substances, it is found that 
the precious stones are harder than any other class of bodies, the diamond 
standing first, and the ruby, sapphire, topaz, and emerald following in order. 
Rhodium and iridium are among the hardest metals, on which account they 
are used for the tips of gold pens. 

36. Tenacity. — Tenacity is that property by which a 
body resists a force that tends to pull it into pieces. 

Both hardness and tenacity are the result of cohesion ; 
but they must not be confounded. Of several rods equally 
thick, that which will support the greatest weight without 
breaking is the most tenacious ; that which it is most diffi- 
cult to cut iuto, is the hardest. 

The metals generally are remarkable for their tenacity. Some, however, 
possess this property in a higher degree than others. This may be shown 
by comparing the weights which different metallic wires of the same size 
are capable of supporting. An iron wire one-tenth of an inch in diameter 
will sustain nearly 550 pounds without breaking, while one of lead will be 
broken by a weight of 28 pounds. 

33. What is Cohesion? What other name has been given to cohesion? What 
is said of cohesion in solids? In liquids? In aeriform fluids? 34. What is 
Adhesion? Describe the experiment with adhesion plates. Describe the exper- 
iment which proves that adhesion operates between solids and liquids. 35. What 
is Hardness ? What is the difference between hardness and density ? In what 
is hardness wanting? How may it be determined which of two bodies is the 
harder ? What bodies are the hardest as a class ? Mention the order in which 
they rank. What two metals are distinguished for their hardness? 36. What 
<s Tenacity? Of what are both hardness and tenacity the result? Show the differ- 



TENACITY. 23 

Iron is the most tenacious of the metals. A cable of this material, com- 
posed of wires one-thirtieth of an inch across, will support the enormous 
weight of 60 tons for each square inch in its transverse section. In conse- 
quence of this great tenacity, such cables are used for the support of suspen- 
sion bridges. 

37. Tenacity of Different Substances. — It is important 
in building and other arts to know the relative tenacity of 
different woods and metals. To determine this, experi- 
ments have been made. Their results do not precisely 
agree, inasmuch as there are differences in different trees 
of the same kind and different pieces of the same metal ; 
yet we may take the following as the average weights that 
can be supported by the several materials mentioned,^ 
taking in each case a rod of given length with a transverse 
section of a square inch. 

Metals.- 





POUNDS. 




POUNDS. 


■Cast Steel, 


134,250 


Woodi 


.—Ash, 14,000 


Swedish Iron, 


72,000 




Teak, 13,000 


English Iron, 


55,800 




Oak, 12,000 


Cast Iron, 


19,000 




Fir, 11,000 


Cast Copper, 


19,000 




Maple, 8,000 


Cast Tin, 


4,700 


Rope, 


one inch around, 1,000 


Cast Lead, 


1,825 


Rope, 


three inches around, 5,600 



It is a curious fact that a composition of two metals may be more tenacious 
than either of them separately. Thus brass, which is made of zinc and cop- 
per, has more tenacity than either of those metals. 

38. The liquids have comparatively little tenacity, yet there is a differ- 
ence in them in this respect. Milk, for instance, is more tenacious than wa- 
ter ; this makes it boil over more readily, inasmuch as its bubbles do not 
break, but accumulate, climbing one upon another till they overtop the ves- 
sel. In like manner, it is on account of their superior tenacity that soap-suds 
will make a lather while pure water will not. 

39. Brittleness. — Brittleness is that property which 
renders a body capable of being easily broken. 

ftnce between them. What is said of the tenacity of the metals ? How may their 
relative tenacity be shown ? Compare iron and lead in this respect. What is said of 
;he tenacity of iron? 37. Explain the fact that experiments for determining the te- 
nacity of different substances show different results. What does the table show ? Of 
the metals mentioned in the table, which has the greatest tenacity ? Which, the 
least ? Of the woods mentioned, which is the most tenacious ? Which, the least ? 
What curious fact is mentioned respecting a composition of two metals? 38. What 
is said of the tenacity of liquids? How do milk and water compare in tenacity V 



24 PROPERTIES OF MATTER. 

Brittleness is the opposite of tenacity, but often charac- 
terizes hard bodies. Glass, which is so hard that it will 
scratch the surface of polished steel, is remarkable for its 

brittleness. 

A substance naturally tenacious may be so treated as to become brittle. 
T hus a bar of iron raised to a high degree of heat, if allowed to cool gradu- 
ally, retains its tenacity, and bends rather than breaks ; but, if suddenly cooled 
by being plunged into cold water, it is made brittle. 

40. Elasticity. — Elasticity is that property by which a 
body, compressed, dilated, or bent by an external force, 
resumes its form when that force has ceased to act. 

Stretch a piece of india rubber ; when you let go the 
ends, they will fly back. Bend a bow ; when the string is 
released, the bow will at once return to its former curve. 
These are familiar examples of elasticity. 

41. The force with which a body resumes its form is called the Force of 
Restitution. Those bodies whose force of restitution brings them back, un- 
der all circumstances, exactly to their original form, are said to be perfectly 
elastic. The only perfectly elastic substances are the ai riform bodies. A 
body of air may be kept compressed for years ; yet, on being freed from the 
compressing force, it will immediately expand to its former dimensions. 

42. Many of the hard and dense solids are highly elastic ; for example, 
steel, marble, and ivory. The soft solids generally, such as butter, putty, &c, 
have little or no elasticity ; there are a few, however, that exhibit it, among 
which are india rubber and silk thread. 

43. The elasticity of steel is increased by making it suddenly contract 
when expanded by heat. This is called tempering, and is effected by raising 
the steel to an intense heat, plunging it in cold water, and keeping it there 
for a certain time. The process is a nice one. At Damascus, in Syria, and 
Toledo, in Spain, it was long performed with peculiar skill, so that the sword- 
blades of those two cities were considered superior to all others. At the 
World's Fair in London, a Toledo sword was exhibited, of such exquisite 
temper that it could be bent into a circle, yet on being released sprung back 
and became as straight as ever. 

44. A compound of two metals may possess a higher degree of elasticity 



Soap-suds and water? 39. What is Brittleness ? Of what is brittleness the opposite ? 
What is said of glass? How may iron be made brittle? 40. What is Elasticity? 
Give some familiar examples. 41. What is meant by the Force of Restitution? 
When is a body said to be perfectly elastic ? What are the only perfectly elastic sub- 
stances? 42. What solids are for the most part elastic, and what not? 43. How is 
the elasticity of steel increased ? What is this process called ? Describe the process 
of tempering. Where was it long done with peculiar skill ? Give an account of the 
Toledo blade exhibited at the World's Fair. 44. What is said of a compound of two 



ELASTICITY. 



25 



than either of them separately. Thus bell-metal is much more elastic than 
either the tin or the copper of which it is composed. 

45. An elastic body, thrown against any hard substance, 
rebounds. An india rubber ball bounds back from a wall, 
to a distance proportioned to the force with which it is 
thrown. In such cases, the ball is flattened at the point of 
contact, but instantly resumes its former shape with such 
force as to drive the ball back. 

To prove this, take two ivory balls (Fig. 12), smear one of Fig. 12. 
them with printer's ink, and suspend them near each other by , 
strings of equal length. Bring them gently in contact, and a 
few particles of ink will adhere to the surface of the clean 
ball : strike them violently together, and a larger spot of ink 
will be found there. This could not happen if the two balls 
were not flattened at the moment of striking. 

46. There is a limit to the elasticity of most bodies, beyond 
which, if compressed, dilated, or bent, they will fail to regain 
their original form. An iron wire, if slightly bent, springs 
back, so that no change of form can be detected ; but not so, 
if violently bent. A continued application of the compressing, 
dilating, or bending force, has the same effect. A bow, if kept 
bent for a long time, will lose its elasticity. For this reason, 
an archer, before putting his bow away, is careful to un- 
string it. 

47. The liquids have but little elasticity. They are 
therefore called Non-elastic Fluids ; while aeriform bodies, 
which possess this property in a higher degree than any 
others, are known as Elastic Fluids. 

48. Malleability. — Malleability is that property which 
renders a body capable of being rolled out or hammered 
into sheets. 

From a piece of copper, a workman with no other instrument than his 
hammer will make a hollow vessel without joint or seam, the malleability of 
the metal preventing it from giving way under his blows. Dough, which 
3an be made into very thin sheets under the rolling-pin, affords a familiar 
illustration of malleability. 

Malleability belongs chiefly to the metals, yet in some of them, such as 
antimony and bismuth, it is wanting. It is strikingly exhibited in silver, 



metals ? Give an example. 45. What does an elastic body do, when thrown against 
a hard substance? In such cases, what takes place ? Prove this by an experiment. 
46. What is said of the limit of elasticity? Give examples. 47. What names have 
been given to liquids and aeriform bodies? Why? 48. What is Malleability 9 Give 



26 MECHANICS. 

platinum, iron, and copper, but most of all in gold. A cubic inch of this met- 
al may be beaten out till it covers 282,000 square inches, which makes the leaf 
on ^y 2F2V o"o °f an i n °k thick. I n other words, it would take 282,000 strips 
of such gold leaf, lying on each other, to make the thickness of an inch. 

49. Ductility. — Ductility is that property which ren- 
ders a body capable of being drawn out into wire. 

The malleable metals are for the most part ductile, but 
Hot always in the same degree. Thus gold exceeds all the 
other metals in ductility as well as in malleability ; but tin, 
which can readily be beaten into very thin sheets, can not 
be drawn out into small wire. 

Gold wire has been made so attenuated that fifty miles of it would weigh 
but an ounce. Platinum, which is nearly as ductile as gold, has been drawn 
into wire only 5 fi ~ 7 of an inch in diameter and invisible to the naked eye. 
Glass, when softened by fire, becomes exceedingly ductile, and may be spun 
out into flexible and elastic threads scarcely larger than the thread of the 
silk-worm, 



CHAPTER III. 

M E C HA N I C S. 

50. Mechanics is that branch of Natural Philosophy 
which treats of forces and their application in machines. 

51. Force and Resistance. — When we see a body be- 
gin to move, cease to move, or change its motion, since it 
can do neither of itself, we know that it has been acted on 
by some external agency, which we call a Force. The 
elasticity of a bow which sends an arrow through the air, 
is a force ; the wind, which changes its direction, is a force ; 
the attraction of gravitation, which brings it to the earth 
and helps to stop its motion, is a force. 



examples. To what does malleability chiefly belong ? Show the extreme malleabil, 
ity of gold. 49. What is Ductility ? What substances are for the most part ductile ? 
What is the most ductile substance known ? What facts are stated, illustrating the 
ductility of gold, platinum, and glass? 

50. What is Mechanics? 51. What is a Force ? Give illustrations. What is th<? 



MOTION. 27 

That which opposes a force is called the Resistance. 
In the above example, the inertia of the arrow is the re- 
sistance. 

Forces may act on bodies so as to produce either Mo- 
tion or Rest. 

Motion. 

52. Motion is a change of place. 

53. Motion is either Absolute or Relative. 

Absolute Motion is a change of place with reference to 
a fixed point. Relative Motion is a change of place with 
reference to a point that is itself moving. 

Two balls are rolled on the floor. The motion of each, as regards the point 
from which it was thrown, is absolute ; their motion with reference to each 
other is relative. 

54. Rest. — Rest is the opposite of motion, and implies 
continuance in the same place. 

Like motion, Rest is either Absolute or Relative. A 
man sitting on a steamer that is moving forward five feet 
in a second, is at rest relatively to the other objects on 
board. To be at rest absolutely, he must walk five feet 
every second towards the stern of the boat. 

Strictly speaking, there is no such thing as absolute rest in any of the ob- 
jects that surround us ; for the earth moves round the sun at the rate of 
about 96,000 feet in a second, and carries with it every thing on its surface. 
Hills, trees, and houses, therefore, though they occupy the same place with 
respect to each other, are really travelling through space with immense ra- 
pidity. Yet as this is the case with ourselves, with the atmosphere, and all 
things about us, we regard an object as absolutely at rest if it has no other 
motion than this. 

55. Velocity. — The Velocity of a body is the rate at 
which it moves. 

This rate is determined by the space it passes over in a 
given time. The greater the space, the greater the velo- 
city. Thus, if A walks two miles an hour, and B four, B's 
"velocity is twice as great as A's. 

Resistance ? What may the action of forces on bodies produce ? 52. What is Motion ? 
53. How is motion distinguished? What is Absolute Motion? What is Eelative 
Motion ? Illustrate these definitions. 54. What is Eest ? Illustrate Absolute and 
Relative Eest. Show that there is really no such thing as absolute rest. 55. What is 



28 MECHANICS. 

56. The relation between the space passed over, the 
time employed, and the velocity, is such, that when two 
are given, we can find the third. 

Rule 1. — To find the velocity of a body, divide the 
space passed over by the time. 

Example. A locomotive goes 120 miles in 4 hours ; what is its velocity? 
—Dividing 120 by 4, we get 30 ; answer, 30 miles an hour. 

Rule 2. — To find the time, divide the space by the ve- 
locity. 

Example. A locomotive goes 120 miles at the rate of 30 miles an hour, 
how long is it on the way? — Dividing 120 by 30, we get 4 ; answer, 4 hours. 

Rule 3. — To find the space, multiply the velocity by 
the time. 

Example, A locomotive goes 4 hours at the rate of 30 miles an hour ; how 
far does it travel ?— Multiplying 30 by 4, we get 120 ; answer, 120 miles. 

57. Table of Velocities. — It may not be uninteresting 
to compare the average velocities of the following moving 
objects : — 



Miles per hour. 

A man walking 3 

A horse trotting 7 

A slow river 3 

A rapid river 7 

A fast sailing vessel 10 

A fast steamboat 18 

A railroad train 25 

A moderate wind 7 

A storm 50 



Miles per hour. 

A hurricane 80 

Sound 764 

A musket-ball, when first 

discharged 850 

A rifle-ball 1,000 

A 24-lb. cannon-ball 1,600 

Earth in its orbit 65,534 

Light 666,000,000 

Electricity (§772) 1,036,800,000 



58. Kinds of Motion. — There are three kinds of mo- 
tion ; Uniform, Accelerated, and Retarded. 

59. Uniform Motion is that of a body which moves over 
equal spaces in equal times. 

Uniform motion would be produced by a force acting once and then 



Velocity ? How is it determined ? 56. What is said of the relation between 
the space, the time, and the velocity ? Give the rule for the velocity, and 
example. Give the rule for the time, and example. Give the rule for the space, 
and example. 57. What is the velocity of a slow river? A rapid river? A mod- 
erate wind? A hurricane? Sound? Light? The electric fluid ? A rifle-ball ? The 
earth in its orbit ? 5S. Name the three kinds of motion. 59. What is Uniform Mo- 



KINDS OF MOTION. 29 

ceasing to act, if the moving body were free from all other influences, for 
its inertia would keep it moving at the same rate. Gravity and the re- 
sistance of the air, however, constantly retard a moving body ; and, there- 
fore, to keep up a uniform motion, a force just sufficient to nullify these re- 
tarding agencies must continue acting. There are very few cases of uniform 
motion either in nature or art. 9 

60. Accelerated Motion is that of a body whose velo- 
city keeps increasing as it moves. It is produced by the 
continued action of a force. 

A ball dropped from a height is a familiar instance of accelerated motion. 
The moment it is let go, the attraction of gravitation causes it to descend. 
Were this force and every other then suspended, the ball would fall to the 
earth with a uniform motion ; but gravity, continuing to act, forces it along 
faster and faster, and thus imparts to it an accelerated motion. 

A body is said to have a Uniformly Accelerated Mo- 
tion, when its velocity keeps increasing at the same rate ; 
when, for instance, it moves two feet in the first second, 
four in the next, eight in the third, &c. 

61. Retarded Motion is that of a body whose velocity 
keeps diminishing as it moves. It is produced by the con- 
tinued action of some resistance on a moving body. 

A ball rolled over the ground, under the continued action of gravity and 
the resistance of the air, moves more and more slowly, till finally it comes to 
rest. This is an example of retarded motion. 

A body is said to have a Uniformly Retarded Motion, 
when its velocity keeps diminishing at the same rate; 
when, for instance, it moves eight feet in the first second, 
four in the next, and two in the third. 

momentum. 

62. The Momentum (plural, momenta) of a body is its 
quantity of motion. 

A ten-pound ball, moving at the rate of 400 feet in a second, may be sup- 
posed to be divided into ten pieces, each weighing one pound. Each piece 
has a motion of 400 feet in a second; and the quantity of motion, or momen- 



tion? Theoretically, how is uniform motion produced? Practically, how is it pro- 
duced ? 60. What is Accelerated Motion ? How is it produced ? Give an example 
of accelerated motion. When is a body said to have a Uniformly Accelerated Motion V 
61. What is Eetarded Motion ? How is it produced ? Give an example. When is a 
body said to have a Uniformly Eetarded Motion ? 62. What is Momentum ? Give 



30 MECHANICS. 

turn, of all ten, that is, of the whole ball, will be ten times 400, or 4,000. 
Hence the following rule : — 

63. Rule. — To find the momentum of a moving body, 
multiply its velocity by its weight. 

Example. What is the momentum of a ten-pound ball, moving at the rate 
of 400 feet in a second ?— Multiplying 400 by 10, we get 4,000 ; answer, 4,000. 

64. When the momenta of different objects are to be compared, their weight 
and velocity must be expressed in units of the same denomination : if tie 
weight of one is given in pounds, that of the other must be in pounds; if the 
velocity of one is so many feet per second, that of the other must be expressed 
in feet per second. If different denominations are given, reduce them to the 
same denomination. 

Thus : A weighs 50 pounds, and has a velocity of 7,200 miles an hour ; B 
weighs 100 pounds, and has a velocity of 4 miles a second. Which has the 
greater momentum ? 

3,600 seconds make an hour ; and if A's velocity is 7,200 miles an hour, in 
a second it will be p^ of 7,200 miles, or 2 miles. 

A's weight 50 multiplied by A's velocity 2 gives A's momentum 100. 
B's weight 100 multiplied by B's velocity 4 gives B's momentum 400. 
Therefore B's momentum is 4 times as great as A's. 

65. Two bodies of the same weight have momenta proportioned to their 
velocities. Thus, if two balls weighing 5 pounds each, move respectively at 
the rate of 20 and 10 miles an hour, then their momenta will be in the pro- 
portion of 20 to 10, or two to one. 

Two bodies moving with the same velocity, have momenta proportioned 
to their weight. Thus, if two balls moving at the rate of 5 miles an hour, 
weigh 20 and 10 pounds respectively, then their momenta will be in the pro- 
portion of 20 to 10, or two to one. 

66. Since momentum depends on velocity as well as weight, it is obvious 
that, by increasing its velocity sufficiently, a small body may be made to 
have a greater momentum than a large one. Thus, a bullet fired from a gun 
has a greater momentum than a stone many times larger thrown from the 
hand. 

On the same principle, a very heavy body, though its motion may be 
hardly perceptible, may have an immense momentum. This is the case 
with icebergs, rendering them fatal to objects with which they come in col- 
lision. 



an example. 63. Kepeat the rule for finding a body's momentum. Give an example. 
64. When the momenta of different objects are to be compared, what is essential? 
Give an example. 65. When two bodies have the same weight, to what are their 
momenta proportioned ? Give an example. When two bodies have the same velo- 
city, to what are their momenta proportioned ? Give an example. 66. How may a 
greater momentum be given to a small body than a large one ? Illustrate this. How 
do you account for the great momenta of icebergs, notwithstanding their slow mo- 



STRIKING FORCE. "31 



Striking Force. 



67. The Striking or Living Force of a moving body is 
the force with which it strikes a resisting substance. 

Striking Force is sometimes confounded with momen- 
tum, but improperly, inasmuch as it is the product of th~ 
weight into the square of the velocity. Two moving bodies 
may have the same momentum, but differ greatly in their 
striking force. 

Thus, the ball A, weighing 200 pounds and moving 2 miles a minute, has 
a momentum of 200 multiplied by 2, or 400. The ball B, weighing 20 pounds 
and moving 20 miles a minute, also has a momentum of 400 (20 multiplied 
by 20). How do they compare in striking force ? That of A is equal to its 
weight 200 multiplied by the square of its velocity, 4, — or 800. That of B is 
equal to its weight 20 multiplied by the square of its velocity 400,- -or 8,000. 
Therefore, though the momenta of the two balls are equal, the striking force 
of B is 10 times as great as that of A ; if both were fired into a bank of moist 
clay, B would penetrate ten times as far as A. 

68. As the velocity of a body increases, its striking 
force increases also, but in a higher degree. 

If, for instance, a train of cars be moving 50 miles an hour, and another 
train of the same weight 10 miles an hour, the striking force of the former 
will not be to that of the latter as 50 to 10, but as the square of 50 is to the 
square of 10, or as 2500 is to 100. The former train would therefore do 25 
times as much damage as the latter to any object with which it came in col- 
lision, or to itself in case of being thrown from the track. This result is borne 
out by facts. 

69. Rule. — To find the striking force of a moving body, 
multiply its weight into the square of its velocity. 

If the striking force of one body is to be compared 
with that of another, see that their weight and velocity are 
in units of the same denomination. 

Example. The stone A, weighing 1 pound, is thrown at the rate of 20 ft. 



tion ? 67. What is meant by the Striking or Living Force of a moving body ? Wbat 
is the difference between a body's striking force and its momentum ? Exemplify tLis 
difference. 68. How does a body's striking force increase, compared with its veloci- 
ty ? Give an example. How is this result borne out ? 69. Give the rule for finding 
the striking force of a moving body. When bodies are to be compared with respect 
to their striking force, how must their weight and velocity be expressed ? Solve the 
example under the rule. 



32 MECHANICS. 

a second. The stone B, weighing 3 pounds, is thrown at the rate of 2,400 ft. 
a minute. Which will penetrate further into a snow-bank ? 

20 times 20 is 400 = square of A's velocity. 

400 X 1 (A's weight) = 400, A's striking force. 

Reduce B's velocity to the same denomination as A's. If B move 2,400 
feet in a minute, in a second it will move ^~ of 2,400 feet, or 40 feet. 

40 times 40 is 1,600 = square of B's velocity. 

1,600 X 3 (B's weight) = 4,800, B's striking force. 

Ans. — A's striking force being 400, and B's 4,800, B will penetrate into 
the snow-bank 12 times as far as A. 

EXAMPLES FOR PRACTICE. 

1. (See Rule 1, § 56.) A fox-hound will run 30 miles in three hours. What 

is its velocity ? 

2. At the battle of Brandywine, Gen. Greene's detachment marched 4 miles 

in 42 minutes, to relieve Gen. Sullivan. With what velocity did they 
move? 

3. At the most flourishing period of its history, ancient Athens was 25 miles 

in circumference. With what velocity would an Athenian have had to 
move, in order to walk round the city in 5 hours ? 

4. A pigeon will fly 100 miles in 2 hours. What is its velocity ? 

5. P walks 2 miles in 30 minutes ; Q walks 4 miles in 2 hours. Which has 

the greater velocity ? 

Remark. — When different denominations are used, they must be reduced to 
the same denomination, as shown in % 64. 

6. The current of a rapid river runs 1,200 feet in 2 minutes ; a horse at a mod- 

erate trot passes over 30 feet in 8 seconds. Which moves with the 
greater velocity ? 

7. (See Bule 2, § 56.) Strabo tells us that ancient Nineveh was 47 miles in 

circumference ; in what time could a person have walked around it, at 
the rate of 10 miles a day ? 

8. The bombardment of Ostend, on the coast of Holland, was heard in Lon- 

don, a distance of 70 miles. There are 5,280 feet in a mile, and sound 
travels at the rate of 1,120 feet in a second. How many seconds after a 
cannon was fired at Ostend, was the report heard in London ? — Ans. 330. 

9. From the base of the Pyramid of Cheops to its top is 704 feet ; how long 

wiD it take a person to ascend it, walking at the rate of 4 feet per second ? 

10. A rifle-ball moves at the rate of 1,000 miles an hour. If it could main- 
tain the same speed, how long would it be in crossing the Atlantic 
Ocean, which is 3,000 miles broad ? 

11. Light moves 185,000 miles in a second; electricity, on a copper wire, 
288,000 miles in the same time. How long before we could see a flash 
of lightning in a cloud 2 miles off, and how long before the lightning, 
conducted by a copper wire, would strike an object by our side? 

12. In the year 1804, the French philosopher Gay Lussac ascended in a bal- 



EXAMPLES FOR PRACTICE. 33 

loon to the height of 4y 2 miles. He came down at the rate of 660 feet in 

a minute ; how long was he in making the descent ? 
13. (See Rule 3, § 56.) Some of the Alpine glaciers move 25 feet annually. 

How far would they move in 4 years ? 
i4. The comet observed by Newton in 1680 moved 880,000 miles an hour. 

How far at this rate would it move in a day ? 
15. Which will pass over the greater space — a hurricane, moving at the rate 

of 80 miles an hour in 4 hours, or a locomotive, going 30 miles an hour, 

in 10 hours? 
16- If the earth moves in its orbit 65,534 miles an hour, and is 365 days, 6 

hours, in completing its revolution, how long is its orbit? 

17. If a ray of light travels 666,000,000 miles in an hour, how far will it go 
in a day ? 

18. (See Rules, %% 63, 69.) A 24-pound cannon-ball moves at the rate of 1,000 
miles an hour. A battering-ram weighing 10,000 pounds moves at the 
rate of 10 miles an hour. How do their momenta compare ? — Arts. As 
24 to 100; that is, the cannon-ball has a little less than one-fourth of the 
momentum of the battering-ram. 

How does the striking force of the above cannon-ball compare with that 
of the battering-ram ; that is, what would be their comparative effect on 
the wall of a fortress ? — Ans. That of the ball would be 24 times as great 
as that of the battering-ram. 

19. An iceberg weighing 50,000 tons moves at the rate of 2 miles an hour. 
An avalanche of 10,000 tons of snow descends with a velocity of 10 miles 
an hour. How do their momenta compare? 

How do they compare in striking force ? 

20. How does the momentum of a 32-pound ball with a velocity of 2,000 miles 
an hour, compare with that of a 16-pound ball with a velocity of 1,000 
miles an hour? 

Which would penetrate further into a bank of moist clay ? 

21. A locomotive weighing 20 tons moves with a velocity of 40 feet a second. 
Another locomotive weighing 25 tons moves at the rate of 4,800 feet in a 
minute. How do their velocities compare ? 

How do they compare in momentum ? 

If the one with the less striking force penetrate 10 feet into a snow- 
bank, how far will the other penetrate ? 

22. A stone weighing 15 ounces is thrown from the hand with a velocity of 
1,320 feet in a minute. A rifle-ball weighing 3 ounces is discharged at 
the rate of 15 miles a minute. How do their velocities compare ? 

How do they compare in momentum ? 

How many times greater is the striking force of the rifle-ball than that 
of the stone? 



34 



MECHANICS. 



CHAPTER IY. 
MECHANICS (CONTINUED). 

LAWS OF MOTION. 

Mathematical Definitions. 

70. Before treating of the laws of motion, it is neces- 
sary to define the mathematical terms used in connection 
with them. 



Fig. 14. 



Fig. 15. 



Fig. 13. l. A Right or Straight Line is one that has the same 

B direction throughout its whole extent ; as, A B. 

2. Parallel Lines are those which have the same direc 
tion ; as, C D, E F. 

3. A Curve Line, or Curve, is one that changes its di- 
rection at every point ; as, G H. 

4. A Circle is a figure bounded by a curve, every point 
of which is equally distant from a point within, called the 
Centre. Fig. 16 represents a circle, and E its centre. 

5. The Circumference of a circle is the curve that 
bounds it; as, A C F B D. 

6. Any part of the circumference is called an 
Arc ; as, A C, C F. 

7. A Diameter of a circle is a straight line drawn 
through the centre, terminating at both ends in the 
circumference ; as, A B. Every circle has an infinite 
number of diameters, all equal to each other. 

8. A Radius (plural, radii) of a circle is a straight line drawn from the 
centre to the circumference; as, ED, EC, E F, E A, EB. Every circle has 
an infinite number of radii, all equal to each other. The radius of a circle is 
just half its diameter. 

9. A Tangent of a circle is a straight line that touches the circumference 




70. What is a Right Line ? What are Parallel Lines? What is a Curve Line? 
What is a Circle ? What is the Circumference of a circle ? What is an Arc ? What 
is a Diameter of a circle? How many diameters has every circle? What is aEadius? 
How many radii has every circle ? How does the radius of a circle compare with its 



MATHEMATICAL DEFINITIONS. 



35 




Fig. 18 




»n a single point, without cutting it at either end when pro- Fig. 17. 

duced ; as, A B, CD. a 

10. The circumference of every circle is divided into 360 
equal parts, called Degrees. One fourth of the circumfer- 
ence contains 90 degrees, and is called a Quadrant. 

11. An Angle is the difference in direction of two straight c 
'.ines that meet or cross each other. 

12. The Vertex (plural, vertices) of an angle is the point at which its sides 
meet ; as, D in Fig. 18. 

An angle is named from the letter at its vertex, if but 
one angle is formed there. Otherwise, it is named from 
the letters on each side and at the vertex, that at the vertex 
being placed in the middle. Thus the angle in Fig. 13 is 
called D; if more than one angle were formed there, it 
would be distinguished as C D B or B D C. 

The size of an angle does not depend on the length of its sides, but sim- 
ply on their difference of direction. We may extend the lines DC, D B, as 
far as we choose, without making the angle D any larger. 

13. When a straight line meets another straight line in such a way as to 
make the two adjacent angles equal, that is, so as to incline no more to one 
side than the other, it is said to be Perpendicular 
to the latter ; and the angle which it makes on either 
side is called a Right Angle. Thus, FEB and F E A 
(being equal) are Right Angles, and the line F E is 
Perpendicular to the line A B. 

A right angle, it will be seen, is measured by 
one fourth of the circumference of a circle, or 90 de- 
grees. 

14. An Obtuse Angle is one that is greater than a 
right angle ; as, F E D in Fig. 19. 

15. An Acute Angle is one that is less than a right angle ; 
as, F E C in Fig. 19. 

16. A Triangle is a figure bounded by three straight 
lines ; as, A B C, Fig. 20. 

17. A Quadrilateral is a figure bounded by four straight 
lines; as, ABC D, Fig. 21. 

18. A Diagonal of a quadrilateral is a straight line 
which joins the vertices of two opposite angles j as, 
A C, D B, in Fig. 21. 

19. A Parallelogram is a quadrilateral whose oppo- 
site sides are parallel ; as, A B C D, Fig. 21. 



Fig. 19, 

F 




Fig. 21. 




diameter? "What is a Tangent of a circle ? How is the circumference of every circle 
divided? "What is a Quadrant? What is an Angle ? What is the Vertex of an an- 
gle? How is an angle named? On what alone does the size of an angle depend? 
When is one line said to be Perpendicular to another ? What is a Eight Angle ? By 
vhat is a right angle measured ? What is an Obtuse Angle ? What is an Acute An 



36 



MECHANICS. 



Fig. 22. 



20. A Rectangle is a quadrilateral whose angles are 
f all right angles ; as, E F G H, Fig. 22. 

21. A Square is a rectangle whose sides are equal ; 
G as, IJKL, Fig. 23. 

22. A Sphere is a solid bounded by a curved surface, 



Fig. 28, 
I J 

a 




all the points of which are equally distant from a point within called 
the centre ; as, A B C D, Fig. 24. 

23. The Axis of a sphere is a straight line 
passing through its centre and terminating in its 
surface, round which it revolves ; as, the straight 
line connecting A and B, in Fig. 24. 

24. The Poles of a sphere are the extremities 
of its axis ; as, the points A, B, in Fig. 24. 

25. The Equator of a sphere is a great circle 
which we imagine to be drawn round it on its 
surface, midway between the poles ; as, the cir- 
cle C D, in Fig. 24. 

26. An Oblate Spheroid is a figure which dif- 
fers from a sphere only in being flattened at its 
poles, like an orange. 

27. A Prolate Spheroid is a figure which differs from a sphere only in be- 
ing lengthened out at its poles, like a lemon. 

28. A Cylinder is a circular body of uniform diameter, the ends of which 
form equal and parallel circles. A lead-pencil, before it is sharpened, is a 
cylinder ; a stove-pipe is a hollow cylinder. 

71. By investigating the principles of motion, Newton 
arrived at three great laws, which have ever since been 
received. 

First Law of Motion. 

12. A body at rest remains at rest, a body in motion 
moves in a straight line with uniform velocity, unless acted 
on by some external force. 

This law follows from inertia. No body has power of itself to move, to 
^ease moving, or to change its direction or velocity. 

73. The air is a powerful agent in stopping motion. This is shown by 
oausing a wheel to revolve on a pivot, first in the air, and then under a glass 



gle ? What is a Triangle ? What is a Quadrilateral ? What is a Diagonal of a quad- 
rilateral ? What is a Parallelogram ? What is a Kectangle ? What is a Square ? 
What is a Sphere ? What is the Axis of a sphere ? What are the Poles of a sphere ? 
What is the Equator of a sphere ? What is an Oblate Spheroid ? What is a Prolate 
Spheroid ? What is a Cylinder ? 71. How many laws of motion did Newton arrive 
at? 72. What is the First Law of Motion? From what docs this law follow? 
73. How may it be shown that the air is a powerful agent in stopping motion ? 



FIRST LAW OF MOTION. 



37 



receiver from which the air has been removed with an air-pump. In the for- 
mer case, the wheel soon ceases to move ; in the latter, it retains its motion 
for a long time. A pendulum (see § 138) will vibrate nearly a day in an ex- 
hausted receiver. 

74. Friction is the resistance with which a body meets from the surface 
on which it moves. The rougher the surfaces brought in contact, the great- 
er the frictiou, and the sooner the moving body will come to rest. A ball 
rolled over a stony road is soon stopped by the obstacles it encounters ; on a 
level pavement it goes much farther, and farther still on a smooth sheet of 
ice. This is because the friction becomes less in proportion as the surface 
on which the ball rolls becomes smoother. 

75. According to this law, every body left free to obey 
the force that set it in motion will move in a straight line. 
We observe few such motions in nature. The planets in 
their orbits, rivers in their channels, rolling waves, and as- 
cending smoke, all move in curves, in consequence of their 
being acted on by other forces, besides those that set them 
in motion. The tendency of the moving body, however, 
is always to continue in a straight line, even when from 
overruling causes it moves in a circle. 

Attach a ball, for instance, to a cord ; and, 
fastening the end of the cord at a point, 0, give 
a quick impulse to the ball. It will be found to 
move in a circle, AB C D, because the cord keeps 
it within a certain distance of the centre. Were 
it not for this, it would move in a straight line. 
Thus, let the cord be cut when the ball is at A, 
and it will be found to move to E in a tangent to 
the circle A B C D. In like manner, at B it will 
fly off" in a tangent to F, and so at C, D, or any 
other point. 

76. The Centrifugal Force. — The force which tends 
to make a body fly from the centre round which it revolves, 
is called the Cen-trif -u-gal Force. 

The opposite force, which draws* a body towards the 
centre round which it revolves, is called the Cen-trip'-e-tal 
Force. 

Magnificent examples of these two forces are exhibited 




74. What is Friction ? On what kind of surfaces does a moving body encounter the 
most friction ? Exemplify this. 75. What is said of the motions that we find in na- 
ture ? Give some instances. What is the tendency of the moving body ? Illustrate 
this with a ball and cord. 76. What is the Centrifugal Force ? What is the Centrip- 



38 



MECHANICS. 



Fig. 26. 



by the planets revolving round the sun in space. At each 
successive point of their orbits, in obedience to the Cen- 
trifugal Force, they tend to fly off in tangents, disturbing 
the harmony of the universe and carrying desolation in 
their path. They are constantly restrained, however, by a 
Centripetal Force equally powerful, the sun's attraction* 
and the result is that they revolve in curves. 

77. Familiar Examples. — Whirl a wet mop rapidly round, 
and drops of water, propelled by this force, will fly off from 
it in straight lines. 

Suspend a glass vessel containing some colored water, by 
a cord passed round the rim, as shown in Fig. 2G. Turn the 
vessel round till the cords become tightly twisted, and then 
suddenly let it go. It will rapidly revolve, and the centrifu- 
gal force will give the water an impulse away from the centre. 
As it can not escape, it will spread up the sides. Should there 
be water enough, it will rise above the top of the vessel, and 
fly off in straight lines. 

We take advantage of the centrifugal force in discharging 
a stone from a sling. The stone is whirled quickly round the 
hand as a centre, which it is prevented from leaving by two 
strings connected with the strap on which it rests. The in. 
stant one of the strings is let go, the centrifugal force carries 
off the stone in a tangent to the circle it was describing. Its 
direction varies according to the point at which the string is let go, as will 
mppear from Fig. 27. Great velocity may be communicated to the stone with 
this simple apparatus. In the hands of the Per- 
sians, the Rhodians, and other ancient nations, 
the sling was a formidable weapon. 

When a wagon turns a corner rapidly, it is 
liable to be upset in consequence of the centrif- 
ugal force. A person sitting in it feels his body 
sway outward, and one who is on his feet has 
to grasp the wagon to avoid being thrown from 
\V / his place. To counteract the effects of the cen- 

\W y ' trifugal force in curves on railroads, the outer 

rail is laid higher than the inner one, as repre- 
sented in Fig. 28. Were it not for this precau- 





*tal Force ? What examples of these two forces does nature furnish us ? 77. How 
may a mop be made to illustrate the centrifugal force ? How does the apparatus rep- 
resented in Fig. 26 illustrate the Centrifugal Force ? Describe the mode in which a 
•tone is discharged from a sling, and explain the principle. What is the effect of tho 
centrifugal force, when a wagon turns a corner rapidly ? How is this effect counter 



THE CENTRIFUGAL FORCE. 



39 




tion, trains moving swiftly round a curve Fig. 28. 

would often be thrown from the track. 

Instinct teaches a horse running rapidly 
round a small circle, to incline his body in- 
ward, that he may counteract the centrifugal 
force. For the same reason, a circus-rider, 
going swiftly round the ring, has to lean to- 
wards the centre. 

Jugglers take advantage of the centrifu- 
gal force to astonish their audiences with a 
striking experiment, represented in Fig. 29. 
A. B is a wheel with a broad rim, or felly. A 
wine-glass partly filled with water is placed 
on the inner surface of the felly, and the wheel 
is then made to revolve rapidly round the 
axle 0. If the proper amount of motion be communicated 
to the wheel, not only will the wine-glass keep its place 
on the felly, but the water also will remain in it, not a {/ 
drop being spilled, even when the glass is at W. Grav- '// 
ity, which, if the wheel were stationary, would at once 
cause both glass and water to fall, is completely nullified 
by the centrifugal force. 

78. Law of the Centrifugal Force. — The 
centrifugal force of a revolving body in- 
creases according to the square of its velocity. If, there- 
fore, the earth revolved round the sun twice as fast as it 
now does, its centrifugal force would be 4 times as great ; 
if 3 times as fast, 9 times as great; if 4 times as fast, 16 
times as great, <fcc. 

This explains why a cord with which a stone is whirled round, as in a 
sling, is more apt to break under a rapid motion than a slow one. Every 
time the velocity is doubled, the strain on the cord is increased fourfold. 

79. Effect of the Centrifugal Force on Revolving Bod- 
ies. — The centrifugal force acts, not only on bodies moving 
in curves, but also on fixed bodies revolving on their own 
axes. 

When large wheels are turned rapidly by machinery, 
the centrifugal force at the circumference becomes an agent 




acted in railroads ? How does instinct teach a horse to counteract the centrifugal 
force? Describe the juggler's trick performed with the aid of the centrifugal force. 
78. What is the law of the centrifugal force ? When is the cord of a sling most apt to 
break, and why? 79. On what, besides bodies moving in curves, does the centrifugal 



£0 



MECHANICS. 



Fig. 30. 




of tremendous power. Unless such wheels are made of 
very strong materials, their cohesion will be overcome by 
the centrifugal force, and they will fly into fragments. Pon- 
derous grindstones sometimes burst, with the most disas- 
trous effects, when too great a velocity is imparted to them. 

Fig. 30 represents a sphere 
revolving on its axis. All parts 
of the surface have to complete 
their revolution in exactly the same 
time ; therefore, as the parts lying 
on the equator CD are further from 
the axis, and have a greater distance 
to go, they must travel faster than 
the rest. Now we have seen that 
the centrifugal force increases with 
the square of the velocity ; and, therefore, at the equator 
CD it will be stronger than at any other part of the sur- 
face. 

Hence the general law : — On a revolving sphere, the 
centrifugal force is greatest at the equator, and diminishes 
from that point till at the poles it wholly disappears. 

Fig. 31. 80. This difference of intensity in the cen- 

trifugal force at different points is shown when 
a sphere of moist clay is made to revolve rapid- 
ly, as on a potter's wheel. The tendency of par- 
ticles on and near the equator to fly off is so great 
that in those parts the sphere bulges out, becom- 
ing proportionately flattened at the poles. 

A similar result is produced in the apparatus 
represented in Fig. 31. Two thin and flexible 
metal hoops are fixed, at right angles to each 
other, on the axis E F, — fastened at the end F, 
but loose at E, so as to admit of their moving 
freely up and down the rod E F. A rapid rotary 
motion being communicated to the hoops, they will assume an oval form, 
bulging out more and more as their velocity is increased. When allowed 
to come to rest, they will rise to their original position at E. 

force act? What is sometimes its effect on large wheels moved by machinery ? What 
is the law of the centrifugal force in the case of revolving spheres ? Explain the rea- 
son of this. 80. What is the effect of the centrifugal force on a sphere of moist clay 
made to revolve rapidly ? Describe the experiment with the apparatus represented 




SECOND LAW OF MOTION. 



41 



81. The centrifugal force, acting as just described, is suprjosed to have 
given the earth its present form. The matter of which our planet is com- 
posed seems at one time to have been soft, and under a rapid rotary motion, 
before becoming solid, it swelled out at the equator and became depressed at 
the poles. The earth thus became an oblate spheroid, the distance from pole 
to pole being about 26% miles less than the equatorial diameter. 

Second Law of Motion. 



82. A given force always produces the same ej 
whether the body on which it acts is in motion or at rest y 
whether it is acted on by that force alone or by others at 
the same time. 

The earth, as it turns on its axis, carries all things on 
its surface with great velocity from west to east ; yet a 
force acting on any object on the surface causes it to move 
in the same direction, and with the same rapidity, as if the 
earth were at rest. 

Let a stone be dropped from the mast-head of a vessel, and it will fall at 
the bottom of the mast, whether the vessel moves or is at rest. 

A person sitting in a wagon throws up an orange and catches it in his 
hand, whether the wagon is moving or not. 

63. Simple Motion. — Mo- 
tion produced by a single force 
is called Simple Motion. 

84. Resultant Motion. — 
Motion produced by the joint 
action of more than one force 
is called Resultant Motion. 

Resultant motion is illustrated with 
the apparatus represented in Fig. 32. 
The ball C is placed on a square frame 
between two upright wires, on each of 
which a ball slides so as to strike C when it descends. Let the ball A drop, 
and it will drive C to D ; this is an example of simple motion. Let the ball 
B drop, and it will drive C to E ; this, also, is simple motion. Let A and B 



Fig. 32. 




in Fig. 31. 81. What is supposed to have been the effect of the centrifugal force on 
the form of the earth ? How does the equatorial diameter of the earth compare with 
the distance from pole to pole ? 82. What is the Second Law of Motion ? Give some 
familiar illustrations of this law. 83. What is Simple Motion? 84. What is Eesult- 
snt Motion ? Describe the apparatus with which resultant motion is illustrated. 



42 



MECHANICS. 



Fig. 33. 




drop at the same instant, and they will drive C to F; this is resultant 
motion. 

85. We have an example of resultant motion in 

a boat (see Fig. 33) which a person attempts to 

row north across a river, while the tide carries it 

^fs " to the east. Each force produces the same effect 

| yQ as if it acted alone ; and the boatman, when he 

has crossed the river, will find himself neither due 

north nor due east of the point from which he 

started, but north-east of it. 

If, in addition to the boatman's efforts and the tide, the wind should blow, 

this also will produce its full effect ; and the boat will exhibit a resultant 

motion produced by the joint action of the three forces. 

86. The Parallelogram of Motion. — If Figures 32 
and 33 be examined, it will be seen that a body acted on 
by two forces moves in a diagonal direction, between the 
lines in which they would separately propel it. 

In Fig. 33, the boatman, starting at A, would row his boat to B ; the tide 
in the same time would carry it to D. When both act, to get the direction 
of the boat and the point it would reach, we must draw the other sides of the 
parallelogram, BC,DC; the diagonal A C will then show the course of the 
boat, and its extremity C the point it would reach. 

87. If the two forces are equal, the body will move in 
the diagonal of a square, that is, directly between the lines 
in which they would carry it. If one is greater than the 

other, the parallelogram must be constructed 
accordingly. 

Let, for instance, the force used by the boatman be twice 

Iff as great as that of the tide. Then by the time he would reach 

U B, the tide would have carried his boat one-half of that dis- 

tance, to D. Completing the parallelogram, as in Fig. 34, and 
drawing the diagonal A C, we find that under the joint action 
of these forces the boat would reach C. 



Fis. 34. 



Third Law of Motion. 

88. Action is the force which one body exerts on an- 
other subjected to its operation. 



85. How may resultant motion be illustrated in the case of a boat? 86. How does a 
body acted on by two forces move ? Illustrate this with Figure 33. 87. If the two 
forces are equal, how will the body move ? If the forces are unequal, how will it 
move? Apply this principle in Fig. 34. 88. What is Action? What is Eeaction? 



THIRD LAW OF MOTION. 43 

Reaction is the counter-force which the body acted 
xpon exerts on the body acting. 

The third Law of Motion is as follows : — Reaction 
is always equal to Action, and opposite to it in direc- 
tion. 

89. Examples of Action and Reaction. — We strike an egg against a table; 
the table reacts on the egg with the same force and in the contrary direction, 
breaking its shell. We push a wagon forward, and feel the reaction in the 
resistance it offers. A bird, when flying, strikes the air downward blows with 
its wings ; the air reacts upward and supports the bird. A rower pulls his 
oar against the water ; the water reacts and drives the boat in the opposite 
direction. A boy fires a gun ; the exploding powder carries forward the 
ball, but the air thus struck reacts on the gun and causes it to recoil against 
the boy's shoulder. Two boats of equal weight, A and B, are connected with 
a rope : a man in A pulls the rope ; action and reaction being equal, not only 
will the boat B move towards him, but the boat A, which he is in, will move 
with the same velocity towards B. 

90. It is reaction. that kills a person who falls from a height on a hard 
pavement. Another, falling the same distance, lights on a feather bed, and 
receives little or no injury ; not because there is less reaction, but because the 
reaction is more gradual, and therefore his body does not receive so great a 
shock. On the same principle, if a steamboat in making her landing is likely 
to strike violently against the dock, the force of the collision is deadened and 
the boat saved from damage by interposing a coil of rope, or some other sub- 
Stance softer than wood. 

Hence also a bullet, which would penetrate a board, will not go through 
a soft cushion, its motion being gradually and not instantaneously opposed 
by the reaction of the cushion. A person may catch a very heavy stone 
without being hurt, if he allows his hand, the instant he catches it, to be car- 
ried in the direction in which the stone was moving, and thus makes the re- 
action gradual. 

91. Reaction often nullifies action. This was the case 
with the man who tried to raise himself over a fence by 
pulling at the straps of his boots. Tug as he might, he 
found that all the upward impulse he could give himself 
was counterbalanced by an equally strong downward im- 
pulse, and that his utmost efforts could not reverse the law 



What is the Third Law of Motion ? 89. Give some familiar illustrations of the third 
law of motion. 90. What is the effect of reaction on a person falling from a height on 
a hard pavement ? What is the effect, if the person falling lights on a feather bed ? 
What causes the difference ? Give another instance of gradual reaction. How may a 
person catch a very heavy stone without being hurt? 91. What is often the effect 



44 



MECHANICS. 



Fi s- 35 - of nature — that action 

and reaction are equal in 
force and opposite in di- 
rection. 

We read of another man no 
less ingenious, who rigged a huge 
bellows in the stern of his sail- 
boat, that he might always be 
able to make a fair wind. On 
trying the experiment, he found 
that with all his blowing he could 
not move the boat an inch ; for 
the reaction of the air on the bel- 
lows kept her back as much as its action on the sail tended to move her for- 
ward. 

92. Action and Reaction in Non-elastic and Elastic 
Bodies. — Action and reaction are always equal, but they 
are exhibited differently in non-elastic and elastic bodies. 
This difference is shown with suspended balls of soft clay 
and ivory, the latter of which are elastic, while the former 
are the reverse. 




Fig. 36. 



Fig. 36 represents two clay or non- 



Jig. 37. 



1 elastic balls. A is raised and allowed to 
fall. If it met with no resistance, it would 
rise to about the same height on the oppo- 
site side. But, encountering B, it imparts 
B to it a portion of its motion, and both move 
on together, as shown in Fig. 37, though only 
half as far as A would have gone alone. The 
reaction of B is clearly equal to the action of 
A ; for the latter loses just as much motion as 
the former gains. 

If the two balls be of ivory, or any other 
highly elastic substance, A will impart the whole of its mo- 
tion to B, and remain stationary after striking ; while B, as 




©O^ 



of reaction? What humorous instance is given of the nullifying effect of reaction ? 
State the case of the man with the sail-boat. 92. In what two classes of bodies are 
action and reaction differently exhibited ? How is this difference shown? What 
does Fig. 36 represent? Show the effect of action and reaction in these non-elastic 



ACTION AND REACTION. 



45 




A0 




shown in Fig. 38, will swing to the same Fig. 38. 

height that A would have reached if unre- 
sisted. Here again the reaction of B, which 
brings A to rest, is evidently equal to the 
action of A, which sets B in motion. 

93. Fig. 39 affords a further illustration of action and reaction in elastic 
bodies. Five ivory balls are suspended by strings of equal length, so as t"> 
fall in front of a graduated arc, with the aid of Fig. 39. 

which the distance they move can be observed. 
Let the first, A, be drawn out and allowed to 
fall. It will impart all its motion to the second, 
and by the reaction of the latter will be brought 
to rest. In like manner, the second imparts its 
motion to the third, and is kept at rest by reac- 
tion ; and so with the third and the fourth. The 
fifth, B, finally receives the motion ; and, there 
being in this case no reaction to stop it, it flies 
off to the same height from which A started. 

94. Reflected Motion. — Reflected Motion is the mo- 
tion of a body turned from its course by the reaction of 
another body against which it strikes. A ball rebounding 
from a wall against which it has been thrown, aflbrds an 
example of Reflected Motion. 

If a body possessing little or no elasticity be thrown against a wall, it will 
rebound but a short distance, if at all. We find the most striking instances 
of reflected motion in the most elastic bodies. Every boy knows that an 
india rubber ball will bound higher than one made of yarn, and that a yarn 
ball will bound higher than one stuffed with cotton. 

95. When a ball is thrown perpendicularly 
against another body, it rebounds in the same 
line towards the hand from which it was thrown. 
Thus, in Fig. 40, if a ball be thrown from F against 
the surface B C so as to strike it perpendicularly 
at A, it will return in the line A F. If thrown 
from D, however, it will glance off on the other 
side of the perpendicular, at the same angle, to E. 
If D were nearer the perpendicular, the line A E would also be nearer to it; 
if it were farther from the perpendicular, A E would be iartuer in proportion. 



Fig. 40. 




balls. What does Fig. 38 represent? 93. Describe the apparatus represented in Fig. 
39, and tell how it operates. 94. What is Eeflected Motion ? Give an example. 
What bodies exhibit reflected motion most strikingly ? 95. When a ball is thrown 
perpendicularly against another body, how does it rebound ? When thrown so as to 
make an angle with the perpendicular, how will it rebound ? Illustrate this with 



±6 MECHANICS. 

96. The angle DAF in Fig. 40, made by the body in 
its forward course with the perpendicular at the point of 
contact, is called the Angle of Incidence. 

The angle E A F, made by the body in its backward 
course with the same perpendicular, is called the Angle of 
Reflection. 

The great law of reflected motion is as follows : — The 
Angle of Reflection is always equal to the Angle of Inci- 
dence. 



CHAPTER V. 
MECHANICS (CONTINUED). 

GRAVITY. 

97. Terrestrial Gravity. — When a stone is let go, we 
all know that it does not fly up in the air or move sideways, 
but falls to the ground. This is owing, as already men- 
tioned, to a universal property of matter. The stone and 
the earth mutually attract each other ; but the earth, being 
vastly superior in size, draws the stone to itself, or in other 
words, causes it to fall. 

The tendency of bodies, when unsupported, to approach 
the earth's surface, is called Terrestrial Gravity, or simply 
Gravity. 

98. Gravitation. — Attraction is universal. It is not 
confined to things on and about the earth's surface, but 
extends throughout space, millions of miles, and is in fact 
the great agent by which the heavenly bodies are kept 
moving in their respective spheres. The earth as certain- 
ly attracts the planet Uranus, at the vast distance of 
3,845,000,000 miles, as it does the falling stone. 



Figure 40. 96. What is the Angle of Incidence ? What is the Angle 01 Reflection T 
What is the great law of reflected motion ? 

97. When a stone is let go, what does it do? To what is this owing? What is 
meant by Terrestrial Gravity ? 98. What is Gravitation ? How far does gravitation 



GRAVITATION. 47 

The attraction subsisting between the heavenly bodies 
is called Gravitation. 

To Sir Isaac Newton the world owes the great discovery of the law of 
Universal Gravitation. Galileo had investigated the subject of terrestrial 
gravity (a. d. 1590), but he did not imagine that any similar force existed 
beyond the neighborhood of the earth. Kepler advanced a step nearer the 
truth, and spoke of gravitation as acting from planet to planet; still he did 
not conceive of its having any effect on the planetary motions. This discov- 
ery, one of the most important that modern science has achieved, was re- 
served for the mighty genius of Newton. Sitting in his orchard one day 
(a. d. 1666), he observed an apple fall from a bough. This simple circum- 
stance awakened a train of thought. Gravity, he knew, was not confined to 
the immediate surface of the earth. It extended to the greatest heights with 
which man was acquainted ; why might it not reach out into space ? Why 
not affect the moon *? Why not actually cause her to revolve around the 
earth? To test these speculations, Newton at once undertook a series of la- 
borious calculations, which proved that the attraction of gravitation is uni- 
versal; that it determines the orbits and velocities of the planets, causes the 
inequalities observed in their motions, produces tides, and has given its 
present shape to the earth. 

99. Three facts have been established respecting gravi- 
tation : — 

1. Gravitation acts instantaneously. Were a new body 
created in space 1,000 miles from the earth, its attraction 
would be felt at the sun just as soon as at the earth, though 
the one would be 91,000,000 miles off, and the other only 
1,000. 

2. Gravitation is not lessened by the interposition of 
any substance. The densest bodies offer no obstacle to its 
free action. Were a body placed on the other side of the 
moon, it would be attracted by the earth just as much as 
if the moon were not between them. 

3. Gravitation is entirely independent of the nature of 
matter. All substances that contain equal amounts of mat- 
ter attract and are attracted by any given body with equal 



extend ? Give an example. By whom was the law of Universal Gravitation discov- 
ered ? What advance had been made towards it by Galileo ? What, by Kepler ? 
Give an account of the circumstances and reasoning that led Newton to this discov- 
ery. What was proved bv his calculations ? 99. What is the first fact that has been 
established respecting gravitation? Give an example. What is the second fact? 
Give an examnle. What is the third fact ? What evidence is there of this ? 100. What 




48 MECHANICS. 

force. The action of the sun is found to be the same on 
all the heavenly bodies. 

100. Direction of Gravity. — If a piece of lead sus- 
pended by a string be left free to move, it will point to- 
wards the earth. This is the case in all parts of the globe. 
Now, as the earth is round, it follows that at two opposite 
points of its surface, the plummet, or plumb-line (as this 

suspended lead is called), will 
point in opposite directions. 
This will be seen from the 
relative positions of A and 
B, C and D, in Fig. 41. The 
lead, therefore, has no ten- 
dency to fall in any particu- 
lar direction as such, but 
takes all directions according 
to the part of the earth's 
surface which it is near. The 
universal law is, that it must point towards the centre of 
the earth. 

It is not because any peculiar attractive power resides in the centre that 
a falling body tends towards that point ; but because, in a sphere, this is the 
result of the attraction of all the particles. The particles on one side attract 
the falling body as much as those on the other ; and consequently it seeks a 
point between them. 

No two plummets suspended in different places have exactly the same di- 
rection, for the lines in which they hang would meet at the centre of the 
earth. At short distances, however, the difference of direction is so slight as 
to be imperceptible, and the plummets seem to point the same way. 

101. It follows that up and down are relative and not absolute terms. 
AVhat is up to a person in New York, is down to a ship a few miles south-west 
of Australia. If a person in a standing position at New York were to be 
carried in a straight line through the earth to its centre, and on in the same 
direction to the opposite side of the earth, he would come out in the Indian 
Ocean south-west of Australia, but would find himself on his head instead of 
his feet. His head, which at New York pointed up, would now point down. 

is a piece of lead suspended by a string called ? How does the plummet always 
point? On what does the absolute position of the plummet depend? Why does a 
falling body tend towards the centre of the earth ? What is said of the difference of 
direction in plummets suspended in different places ? 101. What is said of the terms 
up and down ? Exemplify this. What is the real meaning of up and down ? Why 



LAWS OF GRAVITY. 49 

Down, therefore, simply means towards the centre of the earth, and up away 
from the centre. 

This explains what the unreflecting are sometimes puzzled to account for, 

w hy persons and things on the side of the earth opposite to them do not 

fall off. Regarding themselves as on the upper side, they can not see what 
keeps those on the under side from being precipitated into space. But really 
there is no under side. All things are alike drawn towards the centre ; all 
are kept on the earth's surface by the same force of gravity. 

102. Laws for the Forcf of Gravity. — The force of 
gravity (and the term is here used in its widest sense, in- 
cluding gravitation) depends on two things, — 1. Amount 
of matter ; 2. Distance, — according to the following laws : 

1. The force of gravity increases as the amount of mat- 

ter increases. 

2. The force of gravity decreases as the square of the 

distance increases. 

103. According to the first law, if the sun contained 
twice as much matter as it now does, it would attract the 
earth with twice its present force ; if it contained three 
times as much matter, with three times its present force ; 
&c. Observe, we say if it contained twice as much matter, 
not if it were twice as large ; for it might be twice its 
present size, and yet so rare as to contain less matter and 
attract less strongly than it now does. If there were two 
heavenly bodies, the one of iron and the other of cork, the 
latter, though twice as large as the former, would have less 
attraction because it would contain less matter. 

As already remarked, the earth is so much larger than the bodies near 
its surface that it is not perceptibly affected by their attraction. Even if a 
ball 500 feet in diameter were placed in the atmosphere 500 feet from the 
earth's surface, the earth, being 580 million million times greater than the 
ball, would draw the latter to itself, while it would advance to meet it, less 
than one ninety-six-thousand-millionth of an inch — a distance so small that it 
can not be appreciated. 

The sun is 800 times greater than all the planets put together. It is on 
account of this enormous amount of matter that its attraction is felt by the 
most remote bodies of the solar system at a distance of many millions of miles. 

do not objects on the under side of the earth fall off? 102. On what does the force of 
gravity depend ? Repeat the two laws of gravity. 103. Explain the first law. Why 
is not the earth perceptibly affected by the attraction of bodies near its surface ? Give 
an example. Why is the attraction of the sun so great ? What would be its effect 

i 3 



50 MECHANICS. 

A man carried to the surface of the sun would be so strongly attracted by it? 
immense mass that he would be literally crushed by his own weight. 

104. According to the second law, if the sun were twice 
as far from the earth as it now is, it would attract the latter 
with but \ of its present force ; if three times as far, with 
\ ; if four times as far, with r V, &c. So, if two equal masses 
were situated respectively 5,000 miles and 10,000 miles from 
the earth's centre, the nearer would be attracted not twice, 
but 4 times, as strongly as the more distant. 

105. All bodies on the earth's surface, however small, 
attract each other with sweater or less force according to 
their masses and distance. This attraction, in most cases, 
is absorbed in the far greater attraction of the earth, and 
consequently can not be perceived. In the case of moun- 
tains, however, it is so strong as to have a sensible effect on 
plummets suspended at their base. Instead of pointing di- 
rectly towards the centre of the earth, a plumb-line in such 
a position is found to incline slightly towards the mountain. 

106. Weight. — When a body is supported or prevented 
from following the impulse of gravity, it presses on that 
which supports it, more or less strongly according to the 
force with which it is attracted. This downward pressure 
is called its Weight. 

"Weight is simply the measure of a body's gravity, and is proportioned to 
the amount of matter contained. A ball of iron is heavier than a ball of cork 
of equal size, because it contains more matter. 

Weight being nothing more than the measure of the force with which 
bodies are drawn towards the earth, it follows that, if the earth contained 
twice as much matter as it now does, they would have twice their present 
weight ; if it contained three times as much matter, three times their present 
weight, &c. 

107. Weight above and below the Earth? s Surface. — 
Since the weight of a body is the measure of its gravity, 
and since gravity decreases as the square of the distance 
from the earth's centre increases, it follows that bodies be- 
on a man carried to its surface ? 104. Illustrate the second law with an example. 
105. Why is not attraction exhibited between small bodies on the earth's surface ? 
How is a plummet suspended near the base of a mountain affected ? 106. What is 
Weight? To what is weight proportioned? If the earth contained twice as much 
matter as it now does, how would the weight of objects on its surface compare with 



WEIGHT ABOVE THE EARTH'S SURFACE. 



51 



Fig. 42. 

20,000 miles - 40 pounds 
6 times rurface distance > I/25 surface weight 



come lighter in the same proportion as they are taken up 
from the earth's surface. A mass of iron which at the 
earth's surface weighs a thousand pounds, taken up to a 
height of 4,000 miles, would weigh only 250 of such pounds, 
or one-fourth as much as before. 

The reason of this is clear. The earth 
being about 8,000 miles through, from its 
centre to its surface is 4,000 miles; and 
from its centre to a point 4,000 miles 
above its surface, is 8,000 miles. 4,000 is 
to 8,000 as 1 to 2 ; but the weight at the 
surface would not be to the weight 4,000 
miles above the surface as 2 to 1, but as 
the squares of these numbers, 4 to 1. 
Hence, if it would weigh 1,000 pounds at 
the surface, it would weigh only 1 ' i as 
much, 4,000 miles above the surface. For 
the same reason, it would weigh »/ 9 of 
1,000 pounds at a distance of 8,000 miles 
from the surface ; 1 / 16 , at a distance of 
12,000 miles ; 7 25 , at a distance of 16,000 
miles, &c. These results are exhibited 
in Fig. 42. 

At small elevations, the weight which 
an object loses amounts to but little. Four 
miles above the earth's surface, a body 
weighing 1,000 pounds would become only 
two pounds lighter. Raised to a height 
of 240,000 miles, the distance of the moon 
from the earth, its weight would be re- 
duced to less than five ounces. 

108. If we could go from the 
surface of the earth to the cen- 
tre, we should find a given object weigh less and less as we 
advanced. The moment we descended beneath the surface, 
we would leave particles of matter behind us, and the at- 
traction of these would act in a direction exactly opposite 
to gravity. 



16,000 miles 
4 time* surface distance 



12,000 miles 
1 times surface distant* 



Twice surface distance 




their present weight ? 107. What is said of the weight of bodies taken up from the 
earth's surface ? What would 1,000 pounds of iron weigh, 4,000 miles above the 
earth's surface ? Show the reason of this. What is said of the loss of weight at small 
elevations ? Four miles above the surface, how much would a body weighing 1,000 
pounds lose ? What would be its weight, 240,000 miles fron? the earth ? 108. If we 



52 



MECHANICS. 



Thus, in Fig. 43, let C represent the centre of the earth, and any object 
beneath the surface. All the particles below the line A B attract down^ 
Fig. 43. Fig. 44. 





8,000 miles 
7,000 miles 
6,000 mi 
5,000 miles 
4,000 miles 



Fig. 45. 

1 U, 250 : 



ward, but all above that line attract it upward, and thus diminish its 
weight. 

At the centre of the earth (see Fig. 44) no object would weigh any thing. 
There would be as many particles above the line D E as below it ; and O, be- 
ing equally attracted on all sides, would have no weight. 

109. All bodies carried below the earth's surface would, therefore, become 
lighter as they approached the centre. Their weight at any given number 
of miles below the surface may be found as follows : — 

For 1 mile below, take |^b|- of the surface weight. 

For 2 miles, take £|tto of the surface weight. 

For 100 miles, take |§££ of the surface weight. 

For 1,000 miles, take £M4 of the surface weight, &c. 

4 ° 

110. Laio of Weight. — From the 
above principles the following law of 
nil v« 7*«V p Ave i» nt * s deduced: — All objects weigh 
the most at the surface of the earth: 
ascending from the surface, their 
weight diminishes as the square of 
their distance from the centre in- 
creases y descending towards the cen- 
tre, their weight diminishes as their 
distance from the surface increases. 

Fig. 45 shows the operation of 
this law in the case of an object weigh- 
ing 1,000 pounds at the earth's surface. 

could go from the surface of the earth to the centre, what would we find respecting 
the weight of a given body? What is the reason of this decrease? Illustrate this 
with Fig. 43. What would all objects weigh at the centre ? Show the reason of this 
with Fig. 44. 109. How "nay we find the weight of a given body one mile below the 




WEIGHT IN DIFFERENT LATITUDES. 



53 



111. Weight at different Parts of' the Earth's Surface. 
: — The weight of a body differs at different parts of the 
earth's surface. A mass of lead, for instance, that weighs 
1,000 pounds at the poles, will weigh only 995 such pounds 
at the equator. 

112. This is owing to two causes : — 

1. The equatorial diameter is about 26^ miles longer 
than the polar diameter; and therefore an object at the 
equator is farther from the centre and less strongly at- 
tracted than at any other point. 

2. The centrifugal force, as shown in § 79, is greatest 
at the equator, and therefore counterbalances more of the 
downward attraction there than at any other part of the 
surface, making the weight less. It has been computed, 
that, if the earth revolved 1 7 times as fast as it now does, 
the centrifugal force at the equator would counterbalance 
gravity entirely, and thus deprive all bodies of weight. If 
the earth's velocity were further increased, all things at the 
equator would be thrown off into space. 

113. The general effect of gravity is 
to draw bodies towards the earth ; but 
sometimes it causes them to rise. A 
balloon, for instance, mounts to the 
clouds. This is because it contains less 
matter than a mass of air of the same 
bulk, or, as we say briefly, it is lighter 
than air. Hence the air, acted on more 
strongly by gravity than the balloon, is 
drawn towards the earth under the lat- 
ter, which is thus caused to rise. 

For the same reason, smoke ascends. 
So, if a flask of oil be uncorked at the 



Fig. 46. 




A BALLOON. 



earth's surface ? Two miles? A hundred miles? A thousand miles ? 110. Eepeat 
the law of weight. 111. What is said of the weight of a body at different parts of the 
earth's surface ? Give an example. 112. To what causes is this owing ? What would 
be the result, if the earth revolved on its axis with seventeen times its present velo- 
city ? 113. Show how gravity sometimes causes a body to rise. Give some illustra- 



54 MECHANICS. 

bottom of a pail of water, the water will be drawn down 
below the oil, and force the latter to the top. 

Falling Bodies. 

114. Velocity or Falling Bodies. — If a feather and 
a cent be dropped from a height at the same time, the cent 
will reach the ground some seconds before the feather. 
This fact Aristotle and his successors explained by teaching 
that the velocity of falling bodies is proportioned to their 
weight ; that a body of two pounds, for instance, would 
reach the ground in just half the time required by a body 
weighing one pound. Galileo was the first to correct this 
error (about A. d. 1590). He held that the velocity of fall- 
ing bodies is independent of their weight, and that, if no 
other force than gravity acted on them, all objects dropped 
at the same time from the same height would reach the 
ground at the same instant. 

So startling a proposition was at once condemned by the learned men of 
the day ; but Galileo, convinced of the truth of his position, challenged his 
opponents to a trial. 

The leaning tower of Pisa [pe'-zah], Italy, was chosen as the scene of the 
experiment, and multitudes flocked to witness it. Two balls were produced, 
one of which weighed exactly twice as much as th*> other, and after being 
examined, to prevent the possibility of deception, a., a given signal they were 
dropped. In breathless anxiety the crowd awaited the result, doubting not 
that it would confound the bold youth of six-and-twenty years, who had dared 
to oppose not only the sages of his own time, but also the established opin- 
ion of centuries and the great master Aristotle himself. To their amazement, 
the bold youth was right ; the balls reached the earth at the same instant. 
Unable to credit their own senses, again and again they repeated the experi- 
ment, but each time with the same result. This triumph, though it awakened 
the jealousy of his defeated rivals, and cost Galileo his place as professor of 
mathematics in the university of Pisa, established the fact that gravity causes 
all bodies to descend with equal rapidity, without reference to their weight, and 
that all apparent differences are caused by some other agency. 

115. Resistance of the Air. — The cause of the differ- 



tions. 114. If a feather and a cent be dropped at the same time, which will reach the 
ground first ? How did Aristotle explain this fact ? What was Galileo's opinion on 
the subject ? How was his theory received by the learned men of the day ? Give an 
account of the trial that was made at Pisa. What fact was established by the experi- 



RESISTANCE OF THE AIR. 



55 



ence of velocity in a falling feather and a falling cent is the 
Resistance of the Air. 

This resistance is proportioned to the extent of surface 
which the falling body presents to the air. The surface, 
indeed, may be so extended that gravity can 
hardly overcome the air's resistance; thus, gold 
may be beaten into a leaf so thin that it will be 
3xceedingly slow in its descent, floating for a time 
in the air. 

116. That the resistance of the air causes the difference of ve- 
locity exhibited by falling bodies, may be proved in two ways : — 

1. A piece of paper, a sheet of gold-leaf, or a feather, with its 
surface extended, floats slowly downward ; roll it into a compact 
mass, and it will descend rapidly like a stone. 

2. Remove the air from a high glass tube (see Fig. 47) by 
means of an instrument called the air-pump, to be described 
hereafter. Then, from an apparatus provided for the purpose, 
drop a feather and a cent simultaneously, and they will reach the 
bottom at precisely the same instant. Let in the air and drop 
them, and the feather will be several seconds longer than the cent 
in reaching the bottom. 

1 1 V. The Parachute. — It is the resistance of the 
air that enables a person to descend in safety from 
a balloon at great heights above the earth's surface. A 
parachute, which spreads open like a large umbrella, is sus- 



pended beneath the balloon. Hav- 
ing taken his position in the bas- 
ket-shaped car hanging beneath, 
the aerial voyager fearlessly de- 
taches himself from the balloon ; 
for, though he is borne downward 
by gravity, the force of his fall is 
so broken by the resistance which 
the air offers to the extended sur- 
face of the parachute that he incurs 



Fig. 48. 




A PARACHUTE. 



ment? What was its result to Galileo ? 115. What causes the difference of velocity in 
a falling feather and a falling cent ? To what is the resistance of the air proportioned ? 
How may the air's resistance almost he made to counterbalance gravity ? Give an 
illustration. 116. Prove in two ways that the resistance of the air causes the differ- 
once of velocity in falling bodies. 117. How is a person enabled to descend safely 



56 MECHANICS. 

little danger. To ensure the safety of a common-sized 
man, a parachute must be a| least 22 feet across. Fig. 48 
represents a parachute ; Fig. 46 shows it attached to a 
balloon. 

118. Law of Falling Bodies. — We have found that 
all bodies acted on solely by gravity fall to the earth with 
the same velocity. It is evidently an accelerated velocity ; 
for gravity, which first causes the motion, continues acting. 
In other words, gravity gives a falling body a certain ve- 
locity in the first second of its descent ; still forcing it 
downward, it increases that velocity in the following sec- 
ond ; and so on till it reaches the earth. 

To find the exact spaces passed over in successive sec- 
onds, and the velocity at any given point of the descent, 
was formerly exceedingly difficult, on account of the rapid- 
ity with which falling bodies move, and the want of conve- 
niences for experimenting on them. Even the greatest 
perpendicular heights w^ere inadequate to the purpose, as 
a falling body would reach their base in a few seconds. 
These difficulties are now removed by an ingenious appar 
ratus, called, after its inventor, Atwood's Machine. 

119. Atwood's Machine. — Atwood's Machine is represented in Fig. 49. It 
consists of a pillar, G-, about six feet high, surmounted by a horizontal plate, 
JK ; from which to the base of the stand extends a perpendicular graduated 
scale, C L, divided into feet, inches, and tenths of an inch. The plate J K 
supports a vertical wheel, D, the axis of which, that it may revolve as far 
as possible without friction, rests on four other wheels, a, b. c, d (d, being 
behind the rest, is not seen in the figure). A and B are equal weights, con- 
nected by a cord, which passes over the wheel D. F is a pendulum which 
vibrates once in a second ; and I is a dial-plate and index (like the face and 
hand of a clock) for marking seconds. 

B, having exactly the same weight as A, just counterbalances it. Now 
attach to A a small weight equal to one sixty -third of the combined weight 
of A and B. This slight addition causes A to descend; but as A descends, 
B of course ascends ; and as neither A nor B, being counterbalanced 



from a balloon at a great height? Describe the process. How large must a parachute 
be for a common-sized man ? 118. With what sort of velocity must falling bodies de- 
scend? Why so? What made it difficult formerly to ascertain the velocity, &c, of 
falling bodies ? What apparatus is now employed for this purpose? 119. Describe 
/Uwood's Machine from the plate. Show its mode of operation. How does this ma- 



ATWOOD'S MACHINE. 



57 



each by the other, has any gravity, 
the gravity of the small weight at- 
tached to A, which sets them in mo- 
tion, must be divided into 64 equal 
parts. Hence A with the added 
weight is 64 times as long in descend- 
ing as it would be if dropped freely 
in the air, and the experimenter thus 
has an opportunity of observing its 
velocity at different points, and as- 
certaining the relative distances pass- 
ed over during the successive beats 
of the pendulum. The distances pass- 
ed over in the first, the second, the 
third, and the fourth second, &c, bear 
the same relation to each other, as 
if the bodies were falling freely in 
space. The velocity, moreover, hav- 
ing been greatly diminished, the re- 
sistance of the air becomes so slight 
that it need not be taken into calcu- 
lation. 

120. It is found with Atwood's 
Machine, that, calling the distance 
traversed in the 1st second 1, that 
traversed in the 2d will be 3 ; that 
in the 3d, 5 ; that in the 4th, 7 ; and 
so on in the series of odd numbers. 
The velocity at the end of the 1st sec- 
ond will be a mean between 1 and 3, 
or 2 ; at the end of the 2d, it will be 
a mean between 3 and 5, or 4 ; at the 
end of the 3d, 6 ; at the end of the 
4th, 8 ; and so on in the series of even 
numbers. 

In 1 second a falling body descends 
16 x /i2 feet ; therefore, according to the 
results obtained with Atwood's Ma- 
chine, it has a velocity at the end of the 
1st second of twice I6V12 feet, or Z2 1 / 6 

chine aid the experimenter ? 120. "What is 
found with Atwood's Machine, respecting 
the distances traversed in successive sec- 
onds ? What is the relative velocity at the 
end of successive seconds ? How far does 
a body fall in the first second ? According 



Fig. 49. 




58 MECHANICS. 

feet, per second. In the second second it descends 3 times I6V12 feet, or 48 7 4 
feet, and at its termination has a velocity of 4 times I6V12 feet, or 647 3 feet, 
per second. In the third second, it descends 5 times I6V12 feet, or 80 5 /i2 feet, 
and at its termination has a velocity of 6 times 167i2> or 96V2 feet, per sec- 
ond, &c. 

Now, as to the whole space passed through in any given time. In 1 sec- 
ond, it will be 16Vi2feet; in 2 seconds, by addition (167 12 + 487 4 ), 6473 feet; 
in 3 seconds, (16Vi 2 + 48V4 + 80 5 /i2) 144 3 / 4 feet; in 4 seconds, (I6V12 1" 48 1 /* 
+ 8OV12+ II2712) 2577a, and so on. 

121. These results are summed up in the following rules : — 

Rule 1. — To find the space through which a falling body 
passes during any second of its descent, multiply 16^2 feet 
by that one in the series of odd numbers which corresponds 
with the given second. 

Example. How far will a stone fall in the tenth second of its descent? — 
The series of odd numbers is 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, &c. The tenth 
is 19 ; 167i2 multiplied by 19 gives 3057i2- — Answer, 3057i2 feet. 

Rule 2. — To find the velocity of a falling body at the 
termination of any second of its descent, multiply 16 T V feet 
by that one in the series of even numbers which corresponds 
with the given second. 

Example. What is the velocity of a stone that has been falling ten sec- 
onds?— The series of even numbers is 2, 4, 6, 8, 10, 12, 14, 16, 18, 20. The 
tenth is 20; 167 12 multiplied by 20 gives 321 2 / 3 -— Answer, 3217 3 feet per 
3econd. 

Rule 3. — To find the whole space passed through by a 
falling body, multiply 16 T V feet by the square of the given 
number of seconds. 

Example. How far will a stone fall in 10 seconds ? — Squaring 10 gives 
100; 167i2 multiplied by 100 gives 1,6087 3 — Answer, 1,6087 3 feet. 

122. — Bodies thrown downward. — These rules apply 
to bodies acted on by gravity alone. If a body is thrown 
downward, the force with which it is thrown must also be 
taken into calculation. 

Thus, if a stone be cast from a height with a force that would propel it 50 

to the results obtained with Atwood's Machine, how far will it fall in successive sec- 
onds, and what will be its velocity at the end of each ? 121. Repeat Rule 1, for find- 
ing the space traversed by a falling body during any second of its descent. Apply 
this rule in the given example. Repeat Rule 2, for finding the velocity of a falling 
body. Apply this rule in an example. Repeat Rule 3, for finding the whole distance 
traversed by a falling body. Give an example. 122. To what bodies do these rules 



BODIES THROWN DOWNWARD. 59 

feet in a second, then in the tenth second, instead of falling 3057 1 a feet, as in 
the example under Eule 1, it would fall 50 feet farther, — that is, 355 7 / n feet. 
Its velocity at the end of the tenth second would likewise be obtained by 
adding 50 feet per second to the velocity obtained in the example under 
Eule 2 : 321% + 50 = 8*11%. — To obtain the whole space passed through, add 
to the result obtained by Eule 3, the distance traversed in consequence of 
the velocity originally imparted. A body thrown downward with a veloci- 
ty of 50 feet per second, would, without any aid from gravity, pass through 
500 feet in 10 seconds. Adding this to 1,608% feet, the distance through 
which gravity alone causes a body to fall in 10 seconds, we have 2,108K 
feet for the whole distance traversed in that time by a body thrown down- 
ward with a velocity of 50 feet per second. 

123. In the above examples, no allowance is made for 
the resistance of the air. But even the bodies most favor- 
ably shaped for falling feel the effects of this resistance. 
Experiments in St. Paul's Cathedral, London, show that in 
4i seconds a body falls 272 feet ; whereas, according to the 
principles stated above, it should fall 325 feet. This differ- 
ence, which amounts to nearly one-sixth of the whole dis- 
tance, is owing principally to the resistance of the air. 

124. As the velocity of a falling body increases 32} feet 
every second, it does not take long for it to acquire a tre- 
mendous speed ; and, as the striking force is proportioned 
to the weight multiplied into the square of the velocity, 
it is clear that even a small body, falling any considerable 
distance, may become a very powerful agent. Hence the 
disastrous effects of hail-stones, which have been known to 
injure cattle and break through the roofs of houses, and 
which prove so destructive to the vineyards in parts of 
Southern Europe that the fields have to be protected from 
their visitations. 

125. Ascending Bodies. — As a falling body increases 
in velocity 32| feet every second of its descent, so an as- 
cending body, being acted on by the same force, loses a 

Apply ? If a body is thrown from a height, what must entei into the calculation ? If 
u stone were thrown down with a force that would propel it 50 feet in a second, how 
far would it fall in the tenth second ? What would be its velocity at the end of the 
tenth second? What would be the whole distance traversed in ten seconds ? 123. For 
what must allowance be made in applying these rules ? How great a difference does 
the resistance of the air occasion ? 124. How are the disastrous effects of hail-stones 
accounted for ? 125. What is said of the velocity of an ascending body ? How may 



60 MECHANICS. 

like amount, and will at last be brought to rest. The num- 
ber of seconds during which it will continue to rise is found 
by dividing the number of feet per second with which it 
starts by 32£. 

The height, therefore, which an ascending body reaches, 
depends on the force with which it is projected upward ; 
and, were there no air to resist its progress, it would al- 
ways reach such a height as it would have to fall from in 
order to acquire the velocity with which it started. The 
spaces traversed and the velocities attained during succes- 
sive seconds would be the same in the ascent as in the de- 
scent, only reversed in order. 

Thus, if projected upward with a velocity of 321 2 /3 feet per second, a ball 
unresisted by the air would continue to rise 10 seconds ; because, to attain a 
velocity of 321 2 /3 feet from a state of rest, it would have to fall 10 seconds. 
In the tenth second of its ascent, it would pass through the same distance as 
in the first second of its descent, 16y i2 feet; in the ninth second of its as- 
cent, the same as in the second of its descent, 48 V4 feet; in the eighth second 
of its ascent, the same as in the third of its descent, &c. 

126. According to the principle just stated, a rifle-ball, shot vertically up- 
ward, would descend on whatever it struck with the same force that it had 
when originally discharged. But it does not do so, on account of the resist- 
ance of the air. This resistance prevents the ball from rising as high as it 
otherwise would do by about one-sixth of the whole distance (see § 123), and 
in its descent it again loses nearly one-sixth. The whole loss thus amounts 
to nearly one-third of the velocity, leaving a little over two-thirds remaining. 
Now, to find the proportion between the striking force of the ball when origi- 
nally projected and its striking force on returning to the same point, we must 
square two-thirds. This gives four-ninths ; and thus we find that the ball, on 
returning to the surface, strikes an object with less than half the effect which 
it has immediately on being discharged — a result borne out by facts. 

Projectiles. 

127. A Projectile is a body thrown through the air. An 
arrow discharged from a bow, a bullet from a gun, a stone 
from the hand, are all Projectiles. 

we find the number of seconds that an ascending body will continue to rise ? Were 
It not for the resistance of the air, how great a height would a hody projected upward 
attain? What is said of the spaces traversed and the velocities attained during suo 
cessive seconds ? Exemplify this in the case of a ball thrown upward with a velocity 
of321f feet per second. 126. According to this principle, Avith what force would a 
ball shot vertically upward descend on an object ? Does it do so ? Explain the rea- 



PROJECTILES. 



61 



Every projectile is acted on by three forces : — 

1. The force by which it was thrown. 

2. Gravity, which constantly impels it towards the earth. 

3. The resistance of the air, which tends to bring it to 

rest. 
128. Path of a Projectile. — A projectile maybe thrown 
with such force as to be borne some distance in a straight 
line, without having its direction sensibly altered by grav- 
ity or the air's resistance ; as in the case of a cannon-ball. 
When, however, its velocity diminishes, the joint action of 
these forces causes it to move in a line more or less resem- 
bling the curve called the pa-rab'-o-la. The less the pro- 
jectile force, the sooner does the body deviate from a 
straight line to a curve. 

Fig. 50. 




Fig. 50 shows the path of a stone thrown obliquely from the hand. The 
propelling force sends it in a straight line to A, and would take it on in the 
same direction to B, were it not that, as soon as its velocity becomes suffi- 
ciently diminished, gravity and the air's resistance give it a circular motion 
to C, and finally bring it to the earth at D. 

129. If thrown straight up, a projectile will descend 
in the same line in which it ascended. If discharged hori- 
zontally from a height, it will describe a curve which varies 



son why it does not. 127. What is a Projectile? Give examples. Enumerate the 
forces by which every projectile is acted on. 128. When a projectile is discharged 
with great force, what is its direction for a time ? When its velocity diminishes, How 
does it move ? What projectiles deviate soonest from a straight line ? Illustrate the 
Dath of a projectile with Fig. 50. 129. If thrown straight up, how does a projectile 



62 



MECHANICS. 



in form according to the velocity originally imparted. The 
greater this velocity, the greater the distance the projectile 
will pass through ; but, whatever the distance traversed, it 
will always reach the ground in precisely the same time that 
it would take to fall to the earth from the height at which 
it was discharged. 



Fig. 51. 







E 


Fan 






--■-.. i 






E 


A 


/ - 






B 




J 


MMM 


c 

D 






A 




ml 


*! 



Thus, in Fig. 51, we have a cannon 
planted on a tower at such a height 
that it would take four seconds for a 
ball to fall from it to the ground. 
Dropped from the cannon's mouth, in 
the first second a ball would reach 
A ; in the next, B : in the third, C ; and 
in the fourth, D. Fired from the can- 
non, and acted on by the projectile 
force alone, it would in one, two, three, 
and four seconds, successively reach 
E, F, G, and H. When both forces act, the ball will move in the dotted line, 
reaching at the end of the successive seconds the points I, J, K, and L. The 
ball fired from the cannon will touch the ground at L at precisely the same 
instant that the ball dropped from it will strike the ground at D. 

130. The resistance of the air, which is but slight when a body moves 
slowly through it, becomes a powerful agent as the velocity of the body in- 
creases. A cannon-ball, fired with a velocity of 2,000 feet in a second, would 
go 24 miles before gravity alone would stop it ; whereas, when opposed by 
the air's resistance, as well as gravity, it goes but 3. 

131. A projectile reaches a greater height and remains 
longer in the air, when thrown straight upward, than when 
thrown in any other direction. 

132. Random. — The Random, or Range, of a projectile 
is the distance in a straight line between the points at which 
it begins and ceases to move. 

When thrown perpendicularly upward, a projectile re- 
turns to the point from which it started, and the random 
is naught. The more its course deviates from the perpen- 
dicular the greater the random becomes, until it is thrown 



iescend ? If discharged horizontally from a height, what kind of a line does a pro- 
jectile describe ? What projectiles, so discharged, will traverse the greatest distance? 
How long will it take projectiles discharged horizontally from a height to reach the 
ground ? Explain these principles with Fig. 51. 130. In what case does the resist- 
ance of the air become a very powerful agent? Show this in the case of a cannon 
ball. 131. In what direction must a projectile he thrown, to attain the greatest 



Fig. 52. 




GUNNERY. 63 

at an angle of somewhat less than 40 degrees, from which 
point it again diminishes. Were it not for the resistance 
of the air, a projectile would have the greatest random 
when thrown at an angle of 45 degrees. 

Figure 52 shows the 
course of projectiles 
thrown at different angles. 
The ball which leaves the 
cannon's mouth at an an- 
gle of about 37 degrees 
will be the only one to hit 
the vessel. The two balls 
fired at a greater and a 
less angle will fall short 
of it. 

133. Gunnery. — The laws relating to projectiles form 
the basis of the science of Gunnery. The artilleryman 
must know just at what angle to elevate his gun, and how 
great an allowance to make for gravity and the air's re- 
sistance. 

134. Military projectiles are discharged with the aid of 
gunpowder. This is a solid, which by the application of a 
spark is instantaneously converted into a highly elastic 
fluid, and in that form expands to many times its previous 
bulk. This sudden expansion, confined within a cannon, 
finds vent at its mouth, and with such force as to impart 
great velocity to a ball or other missile. 

Who invented gunpowder can not be ascertained. It was known many 
centuries before the Christian era to the Chinese, who used it for levelling 
hills, blasting rocks, and also, as the remains of ancient pieces of ordnance 
indicate, for military purposes. Other eastern nations appear to have been 
acquainted with its use at an early date. Roger Bacon, the celebrated Eng- 
lish philosopher, in a work written about 1270 a. d., alludes to it as a well 
known composition. Fifty years later, Berthold Schwartz, a Prussian monk, 

height? 132. What is the Random of a projectile? What is the random of a pro- 
jectile thrown perpendicularly upward ? At what angle must a projectile be dis- 
charged, to have the greatest random ? What would be the angle, were it not for 
the resistance of the air? Explain Fig. 52. 133. What science is based on the lawa 
of projectiles? 134. How are military projectiles discharged? Explain the mode in 
which a projectile is discharged with gunpowder. By whom was gunpowder invent- 
ed? To whom was it early known? What English philosopher alluded to it, and 
When ? What Prussian monk investigated its properties ? Where and when were 



64 



MECHANICS. 



Fig. 58. 



investigated its properties ; he has by some been called its inventor, as Ba- 
con has by others. The first that we hear of cannon's being used in war is 
at the battle of Cressy, between the French and English, a. d. 1346. 

135. As the striking force of a body increases with the 
square of its velocity, the pieces of ordnance used in attack- 
ing a fort are so charged as to give the balls the greatest 
possible velocity. In naval engagements, on the other 
hand, no greater velocity is desired than will just plant the 
balls in the enemy's hull ; for thus, imparting the whole of 
their motion to the ship, they give it a greater shock, and 
do more damage by splintering its tim- 
bers, than if they have sufficient veloci- 
ty to carry them completely through. 

136. The Ballistic Pendulum. — 
Several methods have been tried for 
measuring the initial velocity of can- 
non-balls. One is to suspend the piece 
from which the ball is fired, and meas- 
ure its recoil, which is pro- 
portioned to the force with 
which the ball is dis- 
charged. Another method 
is by means of the Ballis- 
tic Pendulum, represented in Fig. 53. 

From a horizontal shaft of iron, is suspended, by strips of wrought 
iron, in such a way as to move freely backward and forward, a hollow 
block of cast iron, A, filled with sand. The ball, fired against an opening 
in the face of the block, and received on a sheet of lead within, drives back 
the block to a distance proportioned to the ball's velocity. This distance 
is measured on a graduated limb, B, placed under the axis of the block, 
by means of a brass slide, C, which is attached to the block and moves on 
the limb. The weight of the block, the distance it is driven, and the weight 
of the ball being known, the velocity of the ball can be determined. 

137. It is found by experiments with the ballistic pen- 
dulum that the greatest velocity that can be given to a can- 
non-ball is a little less than 3,000 feet in a second. To make 




THE BALLISTIC PENDULUM. 



cannon first used in war? 135. How are pieces of ordnance charged for attacking a 
fort? How in naval engagements, and with what object ? 136. What methods have 
been tried for measuring the velocity of balls? Describe the Ballistic Pendulum. 
137. What is the greatest velocity that can be given to a cannen-ball ? What is said 



THE PENDULUM. 



65 



a piece carry the greatest distance, it must be charged with 
a certain amount of powder, which is not uniform, but va- 
ries even in different pieces of the same size. A larger 
charge is not only useless, but dangerous, as it may burst 
the gun. 

The longer the barrel of a gun, the greater is the velo- 
city imparted to the ball ; but its random is thus only 
slightly increased, and, for various reasons, great length is 
now regarded as a positive disadvantage. 



Fig.H 
A 



The Pendulum, 

138. A Pendulum 
consists of a heavy 
ball suspended in such 
a way as to swing to and 
fro. Fig. 54 represents 
a Pendulum. 

If raised on one side and 
let go, the ball of the pendu- 
lum, B, will be carried down 
by gravity with such force as 
to rise by its inertia to the 
same height on the opposite '-•.. 
side. From this point it will ''-•..£ 

again fall and rise on the other 

side ; and, if no other force 
than gravity operated, it would 
keep on rising and falling for- 
ever. The friction at the point of suspension, however, and the resistance 
of the air, are constantly tending to check its motion ; and the consequence 
is that it swings each time a less distance, and finally comes to rest. 

139. When swinging to and fro, a pendulum is said to 
vibrate ; and the portion of a circle through which it moves 
is called its arc. In Fig. 54, CD is the arc of the pendu- 
lum A B. 

140. Laws of Vibration. — First Law. — The vibra- 



B 

THE PENDULUM. 



of the amount of powder to be used for a charge? What is the effect of lengthening 
the barrel of a gun ? 138. Of what does a Pendulum consist ? What takes place when 
a pendulum is raised on one side and let go ? What causes it finally to come to rest? 
189. When is a pendulum said to vibrate t What is meant by the arc of a pendulum ? 

5 



66 MECHANICS. 

tions of a given pendulum are performed in very nearly the 
same time, whether it moves through longer or shorter arcs. 

Thus, in Fig. 34, if the pendulum A B were raised only to E, it would be 
as long in swinging from E to P as from C to D. The shorter the arc, there- 
fore, the slower its motion. It is on this principle that a swing, when first 
set in motion, goes very slowly, but increases in velocity as it is pushed 
higher and higher. 

141. Second Law. — The vibrations of pendulums of 
different length are performed in different times • and 
their lengths are proportioned to the squares of their times 
of vibration. 

One pendulum vibrates in 2 seconds, another in 4. Then the latter will 
be four times as long as the former ; because they will be to each other as 
the square of 2 is to the square of 4,— that is, as 4 is to 16. Hence, to have 
its time of vibration doubled, a pendulum must be made 4 times as long ; to 
have it tripled, 9 times as long ; to have it quadrupled, 16 times as long, &c. 
A pendulum, to vibrate only once in a minute, would have to be 60 times 60, 
that is 3,600, times as long as one that vibrates once in a second, — or a little 
over 2 miles. 

Conversely, the times in which different pendulums vibrate are to each 
other as the square roots of their length. If one pendulum be 16 feet long 
and another 4, the former will be twice as long in vibrating as the latter ; 
for their times of vibration are to each other as the square root of 16 is to 
the square root of 4, — or as 4 to 2. 

142. Third Law. — The vibrations of the same pendu- 
lum are not performed in the same time at all parts of the 
earth's surface / but, being caused -by gravity, differ slight- 
ly, like gravity, according to the distance from the earths 
centre. 

On the top of a mountain five miles high, for instance, a pendulum vibrat- 
ing seconds would make 10 less vibrations in an hour than at the level of the 
sea, because it would be farther from the earth's centre. At either pole, a 
second-pendulum would make 13 more vibrations in an hour than at the equa- 
tor, because it is nearer the centre, the earth being flattened at the poles. 
Hence the vibrations of the pendulum afford a means of measuring heights. 



;40. What is the first law relating to the pendulum ? Illustrate this with Fig. 56. 
141. "What is the second law ? Apply this law in an example. When the lengths of 
iifferent pendulums are known, how can we find the relative times of vibration ? If 
we have two pendulums. 16 and 4 feet long, how will their times of vibration com- 
pare? 142. "What is the third law? What is the difference in the number of vibra- 
tions in a second-pendulum at the level of the sea and at an elevation of five miles ? 
How would the number of vibrations at the pole compare with those at the equator ? 



THE PENDULUM APPLIED TO CLOCK-WORK. 67 

They also confirm what we have learned, that the polar diameter of the earth 
is26lmiles shorter than its equatorial diameter. 

In the latitude of New York, a pendulum, to vibrate seconds, must be 
about 39Vio inches long; whereas at Spitzbergen, in the far North, it must 
be a little over 39 1 / 5 , and at the equator exactlv 39 inches. 

143. Application of the Pendulum to Clock-work. 
— Galileo, to whom science owes so much, was the first to 
think of turning the pendulum to a practical use. Observ- 
ing that a chandelier suspended from the ceiling of a church 
in Pisa, when moved by the wind, vibrated in exactly the 
same time, whether carried to a greater or less distance, he 
at once saw that a similar instrument might be employed 
in measuring small intervals of time in astronomical obser- 
vations. 

To adapt it to this use, it was necessary to invent some 
way of counterbalancing the constant loss of motion caused 
by friction and the air's resistance. This was done by the 
Dutch astronomer Huyghens \hi'-genz\ who in the year 
1656 first applied the pendulum to clock-work. To this 
great invention modern astronomy owes its precision of ob- 
servation, and consequently much of the progress it has 
made. 

144. As a pendulum vibrating seconds, which is over 
39 inches long, would be inconvenient in clocks, it is custom- 
ary to use one that vibrates half-seconds ; which, according 
to the principles laid down in § 141, is one-fourth as long, 
or a little less than 10 inches. 

145. At the same distance from the equator, the same 
elevation above the sea, and the same temperature, a pen- 
dulum of given length will always vibrate in exactly the 
same time, and a clock regulated by a pendulum will 
keep uniform time. If taken from the equator towards the 
poles, the pendulum will vibrate more rapidly, and the clock 

What is the length of a second-pendulum at New York ? At Spitzbergen ? At the 
equator? 143. Who first thought of turning the pendulum to a practical use ? Ke- 
late the circumstance that led him to do so. To enable it to measure small intervals 
of time, what was first necessary ? Who did this, and thus first applied the pendu- 
lum to clock-work ? 144. What is the length of the pendulums generally used in 
slocks ? 145. Under what circumstances will a pendulum always vibrate in the same 



68 



MECHANICS. 



will go too fast. If taken up a mountain, the pendulum 
will vibrate less rapidly, and the clock will go too slow. If 
expanded by the heat of summer (for such we shall here- 
after learn is the effect of heat), the pendulum will also vi- 
brate less rapidly, and the clock will go too slow. 

146. The Gridiron Pendulum. — To prevent a clock 
from being affected by heat and cold, the Compensation 
Pendulum is used. 



Pig. 55. 



GRIDIRON 
PENDULUM. 



One form of the Compensation Pendulum, known as the Grid- 
iron Pendulum, is represented in Fig. 55. It consists of a frame 
of nine bars, alternately of steel and brass. These are so ar- 
ranged that the steel bars, being fastened at the top, have to ex- 
pand downward ; while the brass ones, fastened at the bottom, 
expand upward. The expansive power of brass is to that of steel 
as 100 to 61 ; therefore, if the length of the steel bars is made 
3# %i the length of the brass bars, the expansion of the one metal 
counterbalances that of the other, and the pendulum always re- 
mains of the same length. The steel bars in the figure are rep- 
resented by heavy black lines ; the brass ones, by close parallel 
lines. 

147. A clock is regulated by lengthening or shortening its 
pendulum. This is done by screwing the ball up or down on 
the rod. The ball is lowered when the clock goes too fast, and 
raised when it goes too slow. 



EXAMPLES FOR PRACTICE. 

1. {See Fig. 45, and §§ 107, 109.) What would be the weight (that is, the 

measure of the earth's attraction) of an iceberg containing 40,000 tons of 
ice, if raised to a height of 1,000 miles above the earth's surface ? 
What would it weigh 1,000 miles beneath the earth's surface? 

2. A horse at the earth's surface weighs 1,200 pounds ; what would he weigh 

4,000 miles above the surface? 

How far beneath the surface would he have to be sunk, to nav« the 
same weight?— Am. 3,000 miles. 

3. A Turkish porter will carry 800 pounds ; how many such pounds could he 

carry, if he were placed half way between the surface and the centre of 
the earth, and retained the same strength? — Ans. 1,600. 

How many such pounds could he carry, if elevated 4,000 miles above 
the surface with the same strength? 



time ? What will cause it to vibrate more rapidly, and what less ? 146. To prevent 
a clock from being affected by heat and cold, what is used ? Describe the Gridiron 
Pendulum. 147. How is a clock regulated ? 



EXAMPLES FOR PRACTICE. 69 

I. What would a body weighing 100 pounds at the earth's surface weigh 

1,000 miles above the surface ? 
What would it weigh 1,000 miles below the surface ? 
5. Would an 18- pound cannon-ball weigh more or less, 2,000 miles above the 
earth's surface, than 2,000 miles below it, — and how much ? 

5. At the centre of the earth, what would be the difference of weight between 

a man weighing 200 pounds at the surface and one weighing 100 pounds ? 
Four thousand miles above the surface, what would be the difference 
in their weight ? — Ans. 25 lb. 
T, (See Rule 1, § 121. — In the examples that follow, no allowance is made for 
the resistance of the air.) A man falls from a church steeple; how many 
feet will he pass through in the third second of his descent ? 

6. How far will a stone fall in the twelfth second of its descent ? 

9. (See Rule 2, § 121.) How great a velocity does a falling stone attain in 7 

seconds ? 

10. A hail-stone has been falling one-third of a minute ; what is its velocity? 

II. (See Rule 3, § 121.) How far will a stone fall in 10 seconds? 

12. How far will a hail-stone fall in one-third of a minute? 

13. I drop a pebble into an empty well, and hear it strike the bottom in ex- 
actly two seconds. How deep is the well ? 

How many feet does the pebble fall in the first second of its descent? 
How many, in the second ? 
What velocity has the pebble at the moment of striking ? 

14. A musket-ball dropped from a balloon continues falling half a minute be- 
fore it reaches the earth ; how high is the balloon, and what is the velo- 
city of the ball when it reaches the earth ? 

15. What is the velocity of a stone dropped into a mine, after it has been fall- 
ing 7 seconds, and how far has it descended ? 

16. (See § 122.) What would be the velocity of the same stone at the end of 
the seventh second, if thrown <into the mine with a velocity of 20 feet in 
a second, and how far would it have descended ? 

17. An arrow falls from a balloon for 9 seconds. How far does it fall alto- 
gether, how far in the last second, and what velocity does it attain ? 

What would these three answers be, if the arrow were discharged from 
the balloon with a velocity of 10 feet per second? 

18. (See § 125.) How long will a ball projected upwards with a velocity of 
128% feet per second, continue to ascend? — Ans. 4 sec. 

How great a height will it attain ? — Ans. 257;% ft. 
What will be its velocity, after it has been ascending one second ? 
After two seconds ? After three seconds ? 

19. How many seconds will a musket-ball, shot upward with a velocity of 
225Ve feet in a second, continue to ascend ? 

How many feet will it rise ? 

20. A stone thrown up into the air rises two seconds ; with what velocity was 
it thrown? 

21. (See § 141.) How many times longer must a pendulum be, to vibrate only 
once in a second, than to vibrate three times in a second ? 



70 MECHANICS. 

22. Two pendulums at the Cape of Good Hope vibrate respectively in 40 sec- 
onds and 10 seconds ; how many times longer is the one than the other ? 

23. Two pendulums at New Orleans vibrate in 40 seconds and 10 seconds ; 
how many times longer is one than the other ? 

24. In the latitude of New York, a pendulum vibrating seconds is 397io 
inches in length ; how long must one be, to vibrate once in 10 seconds ? 
— Ans. 3,910 inches. 

How long must one be, to vibrate 4 times in a second at the same place? 
— Ans. 2 71 /i6o inches. 
35. At the equator, a pendulum 39 inches long vibrates once in a second; how 
long must a pendulum be, to vibrate once in half an hour at the same 
place ? 

How long must one be, to vibrate 10 times in a second ? 
26. At Trinidad, a seconds-pendulum must be about 39V50 inches long ; what 
would be the length of one that would vibrate 3 times in a second ? 

What would be the length of one that would vibrate 3 times in a 
minute ? 

What would be the length of one that would vibrate 3 times in an hour ? 



CHAPTER VI. 
MECHANICS (CONTINUED). 

CENTRE OF GRAVITY. 

148. The Centre of Gravity of a body is that point 
about which all its parts are balanced. 

The centre of gravity is nothing more than the centre 
of weight. Cut a body of uniform density in two, by a 
plane passed in any direction through its centre of gravity, 
and the parts thus formed will weigh exactly the same. 
The whole weight of a body may be regarded as concen- 
trated in its centre of gravity. 

149. The Centre of Gravity must be carefully distin- 
guished from the Centre of Magnitude and the Centre of 
Motion. 



148. What is the Centre of Gravity ? How may we divide a body of uniform 
density into two parts of equal weight ? Where may we regard the whole weight of 
ft body as concentrated ? 149. From what must the centre of gravity be carefully 



CENTRE OF GRAVITY. 



71 




150. The Centre of Magnitude (or, as we briefly call it, 
the Centre) of a body, is a point equally distant from its 
opposite sides. 

151. The Centre of Motion in a revolving surface is a 
point which remains at rest, while all the other points of 
the surface are in motion. 

In all revolving bodies, a number of points remain at 
rest. The line connecting them is called the Axis of 
Motion, or briefly, the Axis of the body. 

152. The centre of gravity may coincide Fi S- 56 - 

with the centre of magnitude and lie in the 
axis of motion, but need not do so. In 
Fig. 56, A represents a wheel entirely of 
wood of uniform density ; here the centre 
of gravity coincides with the centre of 
magnitude, C, and both lie in the axis of 
motion. B represents the same wheel 
with its two lower spokes and part of the felly of lead. The centre of 
magnitude, C, still lies in the axis, but the centre of gravity has fallen 
toD. 

When a body is of uniform density, its centre of gravity coincides with 
its centre of magnitude. When one part of a body is heavier than another, 
the centre of gravity lies nearer the heavier part. 

153. A line drawn perpendicularly downward from the 
centre of gravity is called the Line of Direction. In Fig. 
56, CE and D E are the Lines of Direction. 

154. How to find the Centre of Gravity. — The part 
of a body in which the centre of gravity is situated, may be 
found, in some cases, by balanc- 
ing it on a point. Thus the cen- 
tre of gravity of the poker rep- 
resented in Fig. 57 lies directly 
over the point on which it is 
balanced. 

155. In a solid of regular 



Fig. 57. 




distinguished ? 150. What is the Centre of Magnitude ? 151. What is the Centre of 
Motion ? What is the Axis of a revolving sphere ? 152. Show, with Fig. 56, how the 
centre of gravity may not coincide with the centre of magnitude, or lie in the axis. 
When does a hody's centre of gravity coincide with its centre of magnitude ? When 
one part is heavier than another, where does the centre of gravity lie ? 153 What 



72 



MECHANICS. 



Fig. 58. 



shape and uniform thickness and density, so thin that it 
may be regarded as a mere surface, such as a piece of paste- 
board, the centre of gravity may be found by ascertaining 
any two straight lines drawn from side to side that will 
divide it into two equal parts. The point at which these 
lines intersect is the centre of gravity. Thus, in a paralleV 
ooram, the centre of gravity is the point at which its two 
diagonals intersect. 

When such a surface is irregular in shape, sus- 
pend it at any point, so that it may move freely, 
and Vv'hen it has come to rest, drop a plumb-line 
from the point of suspension and mark its direc- 
tion on the surface. Do the same at any other 
point, and the centre of gravity will lie where the 
two lines intersect. 

Thus, suspend the irregular body represented 
in Fig. 58 at the point A ; and, dropping the 
plumb-line A B, mark its direction on the surface. 
Then suspend it at C ; drop the plumb-line C D, 
and mark its direction. The lines cross at E, and 
there will be the centre of gravity. 

156. When two bodies of equal 
weight are connected by a rod, the 
centre of gravity will be in the centre 
of the rod. When two bodies of unequal weight are so con- 
nected, the centre of gravity 
will be nearer to the heavier 
one. These principles are il- 
lustrated in Fig. 59, in which 
C represents the centre of 
gravity. 

157. Stability of Bodies. — The Base of a body is its 
lowest side. When a body is supported on legs, like a 




Fig. 59. 




Is the Line of Direction ? 154. In some bodies, how may the part in which the cen- 
tre of gravity lies be found ? 155. How may the centre of gravity be found, in a thin 
solid body of regular shape and uniform thickness and density ? How may it be 
found in such a solid, when the shape is irregular ? Explain the process with Fig. 58. 
156. When two bodies of equal weight are connected by a rod, where does the centre 
of gravity lie ? How does it lie, when the bodies are of unequal weight ? 157. What 



CENTRE OF GRAVITY. 



73 



chair, its base is formed by lines connecting the bottoms of 
its legs. 

158. When the line of direction falls within the base, a 
body stands ; when not, it falls. 



Fig. 60. 



Fig. 61. Fig. 62. 




Fig. 63. 



In Fig. 60, G is the centre of grav- 
ity ; since the line of direction, Gr P, 
falls within the base, the body will 
stand. In Fig. 61, the line of direc- 
tion falls exactly at one extremity of 
the base, and the body will be over- 
turned by the slightest force. In Fig. 62, the line of direction falls outside of 
the base, and the body will fall. 

A man carrying a load on his back naturally 
bends forward, to bring his line of direction with- 
in the base formed by his feet. Otherwise, the 
line of direction falls outside of the base, as 
shown in Fig. 63 ; and the load, if heavy, may 
pull him over backward. 

159. Of different bodies of the 
same height, that which has the broad- 
est base is the hardest to overturn, because its line 
rection must be moved the farthest to fall outside 

Fig. 64. 




of di- 
of its 




EGYPTIAN PYRAMIDS. 



is the Base of a body ? When a body is supported on legs, how is its base formed? 
158. How must the line of direction fall, for a body to stand? Illustrate this with 
•Figs. 60, 61, 62. What position does a man carrying a load on his back assume, and 
why? 159. Of different bodies equally high, which is the hardest to overturn? 



74 MECHANICS. 

base. Hence a pyramid is the most stable of all figures; 
and, of different pyramids of the same height, that which 
has the broadest base is the most stable. The pyramids 
of Egypt have withstood the storms of more than three 
thousand vears. 

The stability of stone walls is increased by making them broader at the 
base than at the top. Candlesticks and inkstands generally spread out at the 
x>ttom that they may not be easily upset. For the same reason, the legs of 
chairs bend outward as they approach the floor. A three-legged stool or 
table has a smaller base than one that has four legs, and is therefore more 
easily upset. Hence, also, the ease with which a man standing on one leg is 
overturned. 

160. A ball of uniform density has its centre of gravity 
at its centre of magnitude. When such a ball rests on a 
level surface, the line of direction falls on the point of sup- 
port, and it therefore remains in any position in which it is 
placed. But, as the base of a ball consists of a single point, 
— the point in which it touches a level surface, — a slight 
push throws the line of direction beyond the base, and 
causes the ball to roll. 

Fig. 65. 161. When a ball is placed on a 

x— \ sloping surface, the line of direction 
falls outside of the base, and the ball 
begins to roll. A cube placed on the 
same sloping surface maintains its po- 
sition, because the line of direction 
falls within its base. See Fig. 65, in 
which C represents the centre of gravity. 

162. Of different bodies with bases equally large, the 
lowest is the hardest to overturn, because its line of direc- 
tion is least liable to fall outside of its base. 



Wny ? What is the most stable of all figures ? How long have the pyramids of 
Egypt stood ? Give some familiar instances in which the base of a body is madd 
larger than the top, to increase its stability. Why are three-legged chairs and tables 
easily overturned "? 160. In a ball of uniform density, where is the centre of gravity ? 
What is said of the stability of such aball,when resting on a level surface ? 161. When 
such a ball is placed on a sloping surface, what takes place ? Compare it, in this re- 
spect, with a cube. 162. Of different bodies with bases equally large, which is the 



CENTRE OF GRAVITY, 



75 



Fig. 67 





This is apparent from Figs. 66 and 

r>7 The unfinished tower, though 

leaning far over, maintains its upright 
jFi g> 66. position, the 

line of direc- 
tion falling 
within the 
base. When 
made higher 
by the addi- 
tion of seve- 
ral stories, as 

shown in Fig. 67, it will fall, because 

the centre of gravity has been raised, 

and the line of direction now falls outside of the base. 

High chairs for children are unsafe, unless their legs spread at the bot- 
tom. A coach with 

heavy baggage piled ***£• 68 ' 

on its top is in danger 

of upsetting on a 

rough road. On the 

same principle, a load 

of stone may pass safe- 
ly over a hill-side, on 

which a load of hay 

would be overturned. 

Fig. 68 shows that the 

line of direction in the 

one case would fall 

within the base,while 

in the other it would 

fall outside of it. 

163. The lower its centre of gravity, the more stable a 
body is. Those, therefore, who pack goods in wagons or 
vessels, should place the heaviest lowest. 

This principle has been turned to account in building leaning towers. The 
tower of Pisa, which is the most remarkable of these structures, with a height 
of 150 feet, leans to such a degree that its top overhangs its base more than 
32 feet ; yet it has stood firm for centuries. In this case, the centre of grav- 
ity has been brought lower than it would otherwise have been, by the use of 
heavy materials at the bottom and lighter ones higher up. The lower stories 
are of dense volcanic rock, the middle stories of brick, and the upper ones of 




hardest to overturn ? Why ? Illustrate this point with Figs. 66 and 67. Give some 
familiar applications of this principle. 163. Why do those who pack goods in wagons 
I 'nee the heaviest lowest 9 In what has this principle heen turned to account? De- 



76 



MECHANICS. 




Fig. TO. 



an exceedingly porous stone. Thus built, the tower is much less liable to 
fall, than if the same material had been used throughout. 

164. When the centre of gravity is brought beneath the 
point of support, the stability of a body 
is still further increased. 

This is shown in Fig. 69. To balance a needle on 
its point is next to impossible, on account of the 
smallness of the base, and the height of the centre of 
gravity. It may be done, however, by running the 
head of the needle into a piece of cork, C, and stick- 
ing into opposite sides of this cork two forks, A, B, at 
equal angles. The whole may then be poised upon 
the needle's point on the bottom of a wine-glass. In 
this case, the heavy handles of the forks bring the 
centre of gravity below the point of support, in the 
stem of the glass. 

The common toy known as the Rocking 
Horse, represented in Fig. 70, is made on this 
principle. To a horse of any light material, 
bearing a trooper or some other figure, is at- 
tached a wire to which a ball may be fastened. 
When the hind legs of the horse are placed on 
the stand without the ball, the line of direction 
falls outside of the base, and the horse and his 
rider fall. When the ball is attached, how- 
ever, the centre of gravity is brought below 
the point of support ; the horse will then main- 
tain its upright position, and by moving the 
ball may be made to rock up and down. 

165. Effect of Rotary Mo- 
tion. — Rotary Motion, that is, mo- 
tion round an axis, may keep a body from falling, even 
when its line of direction falls outside of its base. Thus, if 
a boy tries to balance his top on its point, he finds it im- 
possible ; but, when he spins it, it stands as long as the ro- 
tary motion continues. The centre of gravity is not over 
the point of support all the time the top is spinning, but is 




ROCKING-HORSE. 



scribe the tower of Pisa, and the materials of which it is built. 164. How is the sta- 
bility of a body further increased ? Show how a needle may be balanced on its point 
by applying this principle. Describe the Eocking Horse, and explain the principle 
involved. 165. What is meant by Rotary Motion ? What is one of its effects ? Why 
does a top fall over when we try to balance it on its point, hut not fall when spinning? 



CENTRE OF GRAVITY IN MAN. 



11 



constantly moving round the axis of motion ; and, before 
the top can fall in consequence of its being on one side of 
the axis, it reaches the other side, and thus counteracts the 
previous impulse. Hence, the faster the top revolves, the 
steadier it is ; as its motion slackens, it gradually reels more 
and more, and finally falls. 

166. Centre of Gravity in Man. — The centre of 
gravity in the body of a man lies between the hips ; the 
base is formed by lines connecting the extremities of the 
feet. A man enlarges this base, and therefore stands more 
firm, when he turns his toes out and places his feet a short 
distance apart. When old and infirm, he enlarges his base 
and increases his stability still further by using a cane. 

When attempting to rise from a sitting position, a man 
must either bend his body forward or draw his feet back- 
ward, in order to bring his centre of gravity over his base ; 
otherwise, he will fall back in making the attempt. So, a 
person who keeps his heels against a wall, can not stoop 
without falling, because he has no room to throw the mid- 
dle of his body far enough back to keep the fine of direc 
tion within the base. 

Fig. 72. 



Fig. 71. 



Nature teach- 
es a man when de- 
scending a height 
to lean backward, 
and when ascend- 
ing to lean for- 
ward, as shown in 
Fig. 71. In like 
manner, when 
carrying a weight 
on one side, we 
sway our body to the other, like the man with the 
watering-pot, in Fig. 72. "We find it easier to 
carry a pail of water in each hand than to carry 
but one, because the weights balance each other, 





166. Where does the centre of gravity lie in a man's hody ? How may a man increase 
his stability ? When attempting to rise from a sitting position, what must a man do ? 
Why can not a person stoop, if he keeps his heels against a wall ? What does nature 
teach a man to do, when descending a height? When ascending a height? When 



78 



MECHANICS. 



and no efiort is necessary to keep the line of direction within the 
base. 

An infant that has not learned to balance itself in a standing position 
creeps on all fours without danger, because it thus brings its centre of grav- 
ity lower and enlarges its base. In order to walk, it must know how to pre- 
serve its balance ; and, as some practice is necessary for this, the child in its 
first efforts is likely to fall. The same is the case with a dizzy or an intoxi- 
cated person, who for the time loses the power of preserving his balance — 
that is, of keeping his line of direction within his base. 

167. When a person slips on one side, he naturally throws out his arm on 
the other. He thus seeks to bring back his centre of gravity over his base, 
and, when he can do so, he saves himself from falling. A person skating has 
to use his arms constantly for this purpose. Rope-dancers, in performing 



Fig. 73. 



their feats, have to shift their centre 
of gravity from point to point with 
great rapidity ; and, finding thei? 
arms insufficient for maintaining 
their balance on the rope, they use 
a long pole, with a slight motion of 
which they can throw the centre of 
gravity into any desired position. 

168. The shepherds of Landes 
[lond], in the south-west of France, 
have turned the art of balancing to 
good account. Having to tend their 
sheep in a region covered with marsh 
in winter and hot sand in summer, 
they mount on stilts about four feet 
high. Though the centre of gravity 
is raised, and their liability to fall 
thus increased, by practising from 
an early age they become exceeding- 
ly expert on these stilts, and can not 
only walk on them, but even dance, 
and run so fast that it is hard for a 
stranger to keep up with them. 

169. Stable and Unstable Equilibrium. — The centre 
of gravity of every body tends to get to the lowest possible 
point. 




SHEPHERDS OF UUTDE8. 



carrying a weight on one side ? Why do we find it easier to carry a pail of water in 
each hand than to carry hut one ? Why is an infant safer when creeping than when 
attempting to walk ? Why does an intoxicated person reel? 167. When a person 
slips on one side, what does he do, and why ? How do rope-dancers preserve their 
balance ? 168. How have the shepherds of Landes turned the art of balancing to 
practical use ? 169. What point does the centre of gravity tend to reach ? Illustrate 



STABLE AND UNSTABLE EQUILIBRIUM. 



79 



A ball suspended by a string, as in Fig. 74, and re- Fig. 74. 

leased from the hand at K, or any other point, will not 
come to rest till it reaches L, because there its centre 
of gravity, B, is at its lowest point. Hence, when a 
pendulum or plummet comes to rest, it always hangs 
vertically. 

A hammer, no matter in what way it is thrown up, 
descends with its iron part first, because the centre of 
gravity, which is in that part, tends to get as low as 
possible. For the same reason, a shuttlecock or an 
arrow, when it has reached its highest point, turns 
and descends with its heaviest part foremost. 

170. A solid body resting on a surface in such a way 
that its centre of gravity is lower than it would be in any 
other position, is said to be in Stable Equilibrium. If its 
centre of gravity could be brought lower by placing it dif- 
ferently, it is said to be in Unstable Equilibrium. 




Fig. 75. 




Thus, the oval body, A B, represent- Fig. 76. 
ed in Fig. 75, is in stable equilibrium, 
because its centre of gravity, C, is at 
its lowest possible point ; and a force 
applied to either end will not cause it 
to fall over, but only to rock to and fro. 
In the position shown in Fig. 76, it is in unstable equilibrium, 
because its centre of gravity might be brought lower ; and a 
slight push will overturn it and bring it to the position shown in Fig. 75. It 
is hardly possible to balance an egg on either end ; but placed on its side, it 
rests securely. 

171. The stability of a sphere, or oval body like an egg^ 
is increased by cutting it into two equal parts, as shown in 




Fig. 77. 



Fig. 17. Bases of this shape are 
used in rocking toys, for support- 
ing the figures of men and animals. 
Of this shape, also, are some of the 
huge Rocking Stones found in different parts of Europe, 
which are so nicely poised that the slightest push causes 
them to rock to and fro, while a dozen men can not over- 
turn them. 



this with Fig. 74 When a pendulum or plummet comes to rest, how does it hang? 
How does a hammer, a shuttlecock, or an arrow, descend, when thrown up into the 
air, and why ? 170. When is a body said to be in Stable Equilibrium ? When, in Un- 
9table Equilibrium ? Apply this in Figs. 75 and 76. 171. How may the stability of a 



80 



MECHANICS. 



172. Paradoxes. — The tendency of the centre of grav- 
ity to reach its lowest possible point sometimes produces 
wonderful effects, or Paradoxes, for which the unlearned 
are at a loss to account. Thus, we know that a ball will roll 
down a sloping surface ; but a ball of light wood may be 
made to roll up a sloping surface by inserting a piece of 
ead in one side. 

Fig. 78. The ball A, for instance, loaded on 

one side with a plug of lead S, is placed 
on a sloping surface. The centre of 
gravity C, which is nearS, at once tends 
to reach its lowest point ; and owing to 
this tendency the ball rolls, till it reaches 
the position shown in B. 

173. In like manner, a double cone, or 
body having the form of two sugar-loaves joined at their large ends, may be 
made to roll up an inclined plane. Fig. 79 represents two rails, joined at one 

end, but apart and somewhat ele- 
vated at the other. Place the 
double cone at the middle of the 
rails. just described, and instead 
of rolling down to the narrow end 
it will roll up to the wide end. 
This is because the centre of grav- 
ity, though apparently going up, is really going down ; for, as the rails di- 
verge, they let the double cone further down between them. 




Fig. 79. 




sphere or oval body be increased ? For what are bases of this shape used ? What 
stones are of this shape ? 172. What are Paradoxes ? How are they sometimes pro- 
duced ? How may a ball be made to roll up a sloping surface ? Explain the principle 
involved, with Fig. 78. 173. Describe the experiment with the double cone, and ex- 
Dlain the principle. 



MOTIVE POWERS. 81 



CHAPTER VII. 

MECHANICS (CONTINUED). 

THE MOTIVE POWER. THE RESISTANCE. THE MACHINE. 

STRENGTH OF MATERIALS. 

1 74. In a previous chapter we have treated of the Laws 
of Motion ; we now proceed to consider the following prac- 
tical points : — 

I. The Motive Power, or Force by which motion is pro- 

duced. 

II. The Resistance to be overcome, or work to be done, 

which is always opposed to the Power. 

III. The Machine, which is used by the Power in over- 

coming the Resistance, when it does not itself di- 
rectly act. 

IV. The Strength of the Materials employed. 

In the case of a steamboat, steam is the Power by which motion is pro- 
duced ; the weight of the boat is the Resistance, which constantly opposes 
the Power. Since steam can not be directly applied in such a way as to move 
the boat, a Machine is used to aid in overcoming the Resistance ; and this 
Machine is the engine. On the strength of the materials employed depend 
the usefulness and safety of the whole. 

Motive Powers. 

175. The chief powers used by man in producing mo- 
tion are gravity, the elastic force of springs, his own 
strength, the strength of animals, wind, water, and steam. 

176. Gravity. — Springs. — Gravity is applied by attach- 
ing weights to machinery, which they keep in motion by 
their constant downward tendency, as in certain kinds of 

174. What four subjects connected with Mechanics are treated of in the presen'; 
chapter ? In the case of a steamboat, what is the power ? What, the resistance ? 
What, the machine ? On what does the usefulness of the whole depend ? 175. Name 
the chief powers employed by man in producing motion. 176. How is gravity ap- 



82 MECHANICS. 

clocks. When the weight descends so far that it reaches a 
support, the machinery ceases to move, and is said to " run 
down". When there is no room to use weights, springs 
are often substituted for them, as in the works of watches. 
A spring is made of steel, or some other elastic substance ; 
which, being bent, produces motion by a constant effort to 
unbend itself. 

177. /Strength of Men and Animals. — With his own 
strength man can produce a certain degree of motion, but 
not such as accomplishes the grandest results. From the 
strength of animals he derives important assistance. Even 
rude nations tame the animals around them, and turn their 
strength to account. The American Indians, when first 
discovered, had not learned to do this ; and therefore, like 
other savages who rely entirely on their own strength, they 
had made no great advance in agriculture, manufactures, 
or any other branch of industry. 

The horse is the animal whose strength is most widely 
and advantageously used. For continued labor, one horse 
is considered equal to five men. A horse of average strength 
can draw a load of a ton, on a good road, from 20 to 25 
miles a day. 

178. Wind and Water. — Still more powerful forces are 
found in wind and water, which are extensively used as 
moving powers by all civilized nations. 

The wind is brought to bear, not only on the sails of vessels, but also in 
mills used for grinding grain, sawing wood, raising water, expressing oil 
from seeds, &c. Such machines are called Wind-mills ; they were introduced 
into Europe from the East, about the time of the Crusades. The great objec- 
tion to the wind as a moving power, is its irregularity, for in still weather the 
machines it moves are useless. 

Water is a very powerful and useful agent. A little stream is often a 

plied ? When is the machinery said to run down ? When there is no room to use 
weights, what are often substituted for them ? How does a spring produce motion ? 
177. What is said of the strength of man as a source of motion ? What, of the 
strength of animals ? What animal is most widely used ? To how many men is one 
horse considered equal ? As regards drawing, what is a day's work for a horse of av- 
erage strength ? 17S. What sources of motion are still more powerful ? How is the 
wind brought to bear ? What are machines moved by the wind called ? Whence 
and when were wind-mills introduced into Europe ? What is the great objection to 



STEAM, AS A MOTIVE POWER. 8-3 

source of prosperity and wealth to an extensive region. Affording what is 
called " water-power ", it moves huge machines, and thus affords the means 
of manufacturing easily and cheaply. Water was first used as a motive power 
by the Romans, in simple machines for grinding grain, about the commence- 
ment of the Christian era. It is now applied in various kinds of machines 
for sawing, spinning, weaving, grinding, &c. Though a stream may run so 
high in spring and so low in summer as to be useless fov a time, there is far 
less difficulty from these causes than from the irregularity of the wind. 

179. Steam. — The greatest of all the powers employed 
by man is steam, or the vapor generated by submitting 
water to a high degree of heat. Steam being an elastic 
fluid, its properties and applications will be considered 
hereafter. 

180. The uses of steam were unknown to the ancients ; it was not till near 
the close of the seventeenth century that its importance began to be realized. 
Its application to machinery marks an era in the world's history, and has in- 
vested man with immense power over matter. Driving the boat and car, it 
bears him what was once a day's journey in an hour. Applied in countless 
varieties of machines, it is the means of supplying us with thousands of com- 
forts unknown to our forefathers. The farmer is indebted to it for his spade, 
hoe, rake, scythe, ploughshare, and all his implements. It helps to make 
the shears with which he cuts the wool from his sheep, and then cards the 
wool, and weaves it into cloth. It separates his cotton from its seed, and 
turns it into muslin and calico. It aids the builder by making his tools, forg- 
ing his nails and bolts, moulding his ornaments, polishing his marble, cutting 
his stone, and sawing his wood. It supplies our parlors with furniture, our 
kitchens with cooking utensils, our dining-rooms with glass and china, knives 
and forks. It knits, twists, washes, irons, dyes, gilds, grinds, digs, and 
prints ; and hardly any work of art meets our eyes, in making which steam 
nas not been directly or indirectly used. It does all this, moreover, with 
wonderful precision and rapidity. The pyramids of Egypt, we are informed, 
kept 100,000 men at work twenty years in their erection. It has been com- 
puted that one powerful steam-engine would have done as much work in the 
same time as 27,000 of these Egyptians. 

The Resistance. 

181. Whatever opposes the Power is called the Resist- 
ance. 

the wind as a moving power ? What is said of water-power ? By whom and when 
was it first used? For what purposes is it now employed ? What are the disadvan- 
tages of water as a moving power ? 179. What is the greatest of the powers em- 
ployed by man ? What is Steam ? 180. When did its importance begin to he real- 
ized ? What has been the result of its application to machinery ? Enumerate the 
different articles which steam is constantly employed in producing. What interest- 



84 MECHANICS. 

182. The resistance is not always of the same character. 
It may be a weight to be lifted, as a pail of water from a 
well ; or a body to be moved onward, as a train of cars ; 
or a wheel to be turned, as in a mill ; or particles to be 
compressed, as in packing cotton in bales ; or cohesion to 
be overcome, as in splitting a log of wood. As the most 
usual form in which the resistance appears is that of a 
weight to be moved, the term Weight is often used instead 
of Resistance, with reference to work of any kind, or what- 
ever opposes the moving power. 

183. Units of Work. — The efficiency of a force is estk 
mated by the resistance it can overcome, or the amount of 
work it can do. In order to compare different forces, we 
must have a uniform unit of work. 

The unit of work adopted is the resistance encountered 
in raising one pound through the space of a foot. Hence, 
to raise a body any distance constitutes as many units of 
work as there are pounds in the body multiplied by the 
number of feet in the given distance. To raise 2 pounds 
of water from a well 6 feet deep, is equivalent to twice 6, 
or 12, units of work. To lift a load of 1,000 pounds 10 
feet involves 10,000 units of work. 

184. Horse-powers. — In estimating large amounts of 
work, it is customary to use horse-powers as a measure. A 
horse can perform 33,000 units of work, that is, can raise 
33,000 pounds a foot, in a minute. An engine, therefore, 
that can perform 33,000 units of work in a minute is said 
to be an engine of one horse-power ; one that can do 66,000 
units of work in a minute is an engine of 2 horse-powers ; 
and so on. Hence the following 

Rule. — To find the horse-power of an engine, divide the 
number of pounds it is capable of raising one foot in a min- 
ute by 33,000. 

:ng fact is stated with respect to the pyramids of Egypt? 181. What is the Resist- 
ance ? 182. Mention some of the different forms in which the resistance appears, and 
give examples. What term is often used instead of resistance, and why ? 183. How 
Is the efficiency of a force estimated ? To compare different forces, what is it neces- 
sary to have ? What is the unit of work generally adopted ? Give examples. 



FRICTION. 85 

185. Friction. — The effect of the moving power is often 
diminished by Friction. 

Friction is the resistance which a moving body meets 
with from the surface on which it moves. 

If all surfaces were perfectly smooth, there would be no friction ; but even 
those bodies that seem the smoothest are really covered with minute projec- 
tions and depressions. These fit into each other, and a certain degree of 
force is required to raise the projections of the one surface over those of the 
other. With the naked eye we can not detect any unevenness on plate glass 
or polished steel ; yet, if we view either through a microscope, we find that 
its surface is far from smooth, and hence there is some friction even when 
these substances are rubbed together. 

186. Friction opposes motion in two ways: — 

1. By increasing the resistance, as when a weight is 
dragged over the ground. 

2. By diminishing the force before it is applied to the 
resistance ; as in machinery, which sometimes loses as much 
as one-third of its power by the rubbing of its different parts 
against each other. 

In estimating the working power of a machine for practical purposes, it 
is necessary to make allowance for the loss occasioned by friction ; but, in 
merely investigating the principles of Mechanics and the construction of ma- 
chines, we proceed as if the surfaces concerned were perfectly smooth, and 
no such thing as friction existed. 

187. Kinds of Friction. — There are two kinds of fric- 
tion : — 

1. Sliding Friction, produced when a body slides on a 

surface, like the runners of a sleigh. 

2. Rolling Friction, produced when a body rolls on a 

surface, like the wheels of a wagon. 

188. Between any given surface and moving body, slid- 
ing friction is much greater than rolling friction. Hence 
we roll a barrel of flour over the ground instead of drag- 



184. How are large amounts of work estimated ? What is meant by a horse-power t 
Give an example. How may the horse-power of an engine be found ? 185. By what 
is the effect of the moving power often diminished ? What is Friction ? How is 
it that friction is exhibited even between surfaces that appear smooth ? Give an ex- 
ample. 186. In how many ways does friction oppose motion ? Mention them. When 
Is it necessary to make allowance for friction, and when not ? 187. How many kinds 
of friction are there? Nam* them, and tell how each is produced. 188. Between any 



86 



MECHANICS. 



ging it, and place a weight that is to be moved in a cart, or 
suspend it between wheels, instead of harnessing a horse 
directly to it. 



Fig. 



On the same principle, we place rollers under a block of marble, and fasten 
castors, or small wheels, to the legs of heavy pieces of furniture. Rollers are 

also used with advantage in pushing 
a ponderous packing-box up an in- 
clined plane into a cart, as shown in 
Fig. 80. In all these cases, sliding 
friction is converted into rolling, and 
the resistance is thus diminished. The 
larger the wheels and rollers employ- 
ed, up to a certain limit, the greater 
the gain ; but even small ones mate- 
rially lessen the friction. 

Rolling friction, on the other hand, 
may be converted into sliding. This is done when the wheels of a heavily 
loaded stage or wagon descending a steep hill are locked, that is, prevented 
from turning by an apparatus provided for the purpose. The resistance is 
thus increased to such a degree that the load can descend in safety. On the 
same principle, brakes are applied to the wheels of cars, to stop them the 
sooner. 

189. Laws of Friction. — Several important laws relating to friction have 
been settled by experiments. In making these, the apparatus represented in 




Fig. 81. 



Fig. 81 has been used. D E is a table, 
on which rests the block C. A string, 
passing over the pulley B, connects this 
block with a scale, A. By putting 
weights in the scale till the block moves, 
we are enabled to measure its friction ; 
and, by making the block of different 
materials, varying its size and surface, 
and allowing it to remain a longer or 
shorter time on the table, the following 
laws have been established : — 

1. The friction of a body is greater when it commences 
moving than after it has been moving for a time. Thus it 




given surface and moving body, how does sliding friction compare with rolling fric- 
tion? Mention some familiar cases in which we convert sliding into rolling friction, 
to lessen the resistance. What is said of the size of the wheels and rollers employed? 
In what cases is rolling friction converted into sliding? 189. How have the facts re- 
lating to friction been settled ? Describe the apparatus employed for this purpose. 
When is the friction of a body greatest ? Between what bodies and surfaces is fric 



LAWS OF FRICTION. 87 

takes a heavier weight to start the block C than it does af- 
terwards to keep it in motion. 

2. Friction is greater between soft bodies than hard 
bodies, and between rough surfaces than smooth ones. A 
sled that can hardly be moved over a newly ploughed field, 
is drawn without difficulty over a frozen pond. 

3. In many cases, friction is increased by letting the 
surfaces remain in contact. At the end of five or six 
days, it has been found to be fourteen times as great as at 
first. 

4. Between the same surfaces, friction is proportioned 
to the weight of the moving body. The friction of a block 
weighing 20 pounds is twice as great as that of a ten-pound 
block. 

5. Within certain limits, friction is not increased by ex- 
tent of plane surface. As long as the weight of a body re- 
mains the same, its friction will not vary, whether it rests 
on a larger or smaller base. In Fig. 81, the block C has its 
upper side hollowed out, so that, if turned over, it will rest 
merely on two ridges ; yet the friction will be the same when 
it rests on that side as on the other. 

190. Modes of Lessening Friction. — No means has yet 
been found of doing away with friction altogether; but it 
may be lessened in three ways : — 

1. By smoothing and polishing the surfaces. 

2. By putting grease or some other lubricant, as it is 
called, between the surfaces. This fills up their depressions. 
Finely powdered plumbago (the common black-lead used 
in pencils), dry for wooden surfaces and mixed with grease 
for metallic ones, is one of the best articles used for this 
purpose. The wood-sawyer greases his saw to make it 
move easily, and cartmen and carriage-drivers keep the 



tion greatest ? In many cases, how may friction be increased ? Between the same 
surfaces, to what is friction proportioned ? What effect is produced on the friction of 
a body by increasing its surface ? Exemplify this with the figure. 190. Can friction 
be entirely removed ? In how many ways may it be lessened ? What is the first of 
these ? What, the second ? What article makes one of the best lubricants ? By 
whom are lubricants used ? How may the friction of a wheel be diminished ? What 



88 



MECHANICS. 




FRICTION WHEEL8. 



Fi s- 82 - axles of their wheels well covered 

with some lubricating preparation. 

3/ The friction of a wheel may 
be diminished by making its axle, 
that is, the cylinder running through 
the centre, turn on the circumfer- 
ences of two other wheels at each 
end, as shown in Fig. 82. Such 
wheels are called Friction Wheels. 
They are used in delicate machinery. 

191. Uses of Friction.— Though friction occasions a great loss of power, 
it is not without its beneficial effects. A river is prevented from rushing 
madly through its channel by the friction of its waters on its banks and bed. 
A tempest gradually loses its force by the friction of the air against the pro- 
jections on the earth's surface. It is friction that prevents the fibres of wool, 
hemp, and cotton, when twisted together, from slipping on each other and 
giving way. Without friction nails would be useless, for they would draw 
right out; the driving-wheels of a locomotive would turn on the rails 
without moving it forward ; and neither man nor beast could walk. It is 
the friction of our feet on the ground that enables us to take steps: when 
the friction is lessened, as on smooth ice, we walk with difficulty ; were 
there no friction, we should find it impossible to walk at all. 



Machines. 

192. Machines are instruments used to aid the Power 
in overcoming the Resistance. 

193. Simple machines used by the hand, are called Tools ; 
as, the chisel, the saw. 

194. Machines of great power are called Engines ; as, 
the steam-engine, the fire-engine. 

195. Machines merely aid the power in its action ; they 
can not create power. This follows from the inertia of mat- 
ter. The mightiest engine, therefore, remains at rest until 
acted on by some motive power ; and, when thus acted on, 
it can not increase the power in the smallest degree, but on 



nre such wheels called ? 191. Mention some of the beneficial effects of friction. 
192. What are Machines ? 193. What are Tools ? 194 What are Engines ? 195. What 
do machines merely do? Why can not a machine increase the power? Illustrate 
this principle in the case of a man who can raise 100 pounds of coal a minute from a 



PERPETUAL MOTION. 89 

the other hand diminishes it, more or less according to the 
friction of its parts. 

If a man standing over a pit 100 feet deep can, in the space of a minute, 
just pull to the top a tub containing 100 pounds of coal, no machine can ena- 
ble him to raise a single pound more in the same time. By using pulleys, he 
may, to be sure, raise 600, 800, or 1,000 pounds at a time, but it will take him 
6, 8, or 10 times as long as before ; and, therefore, in the same time he will do 
no more work than with his hands alone — but less, on account of the friction 
of the pulleys. So, a certain amount of steam, just capable of performing 
50,000 units of work in a minute, can not by any machinery be made to per- 
form a single additional unit of work in the same time. Hence the great uni- 
versal law which follows : — 

196. What a machine gains in amount of work, it loses 
in time; and what it gains in time, it loses in amount of 
work. 

Let us apply this law. A quantity of steam capable of moving 50,000 
pounds a foot in a second, may be made to move 100,000 pounds a foot, but 
it will be two seconds in doing it ; or it may move the weight a foot in half a 
second, but in that case it will move no more than 25,000 pounds. Under no 
circumstances can there be a gain in units of work without a corresponding 
loss of time, or a gain in time without a corresponding loss of units of work. 

197. Perpetual Motion. — By Perpetual Motion is 
meant the motion of a machine, which, without the aid of 
any external force, on once being set in operation, would 
continue to move forever, or until it wore out. 

Such a machine many have tried to invent, but without 
success. Friction and the resistance of the air are con- 
stantly opposing the action of machinery ; and as matter, 
on account of its inertia, can generate no power that will 
compensate for this loss, every machine must in time come 
to rest, unless some external force, such as wind, water, or 
steam, keeps acting upon it. Hence Perpetual Motion is 
impossible. 

198. Advantages of using Machinery. — If no addi- 
tional power is generated by machinery, but there is an 
actual loss from the friction of its parts, why is it employed ? 
— Because in other respects its use is attended with impor- 
tant advantages, among which are the following : — 

pit 100 feet deep. Give another illustration. 196. What is the great universal law 
■>t machines? Apply this law practically. 197. What is meant by Perpetual Mo 



90 



MECHANICS. 



Fig. 83. 




1. Machinery enables us, with a certain amount of pow- 
er, by taking a longer time, to do pieces of work that we 
could not otherwise do at all. 

Thus, a farmer with a crow-bar, as 
shown in Fig. 83, can move a rock which 
with his hands alone he could not stir. 
With the aid of two other men, he could 
carry it or push it where he wanted, in 
one- third of the time that he could move 
it there alone with the crow-bar ; but he 
may not have two others at hand to help 
him. 

With machinery 10 men may do the 
work of 1,000. Of course it will take 
them 100 times as long ; but this loss of time is of little consequence, com- 
pared with the difficulty of getting a thousand men together and placing them 
so as to work without interfering with each other. Some heavy pieces of 
work are of snch a nature that but few laborers can get around them at a 
time ; in these cases, unless the work can be divided, which is not always 
possible, it must remain undone without the aid of machinery. 

2. Machinery enables us to use our power more con- 
veniently. 

The farmer removes a rock from his field with less difficulty and fatigue 
by means of a crow-bar than if he stooped over to lift it with his hands. The 
porter with his block and tackle hoists a box of goods to a loft with far greater 
ease than he could push or carry it up. The apparatus he uses enables him 
to hoist the load by pulling down upon a rope, and when pulling down his 
weight aids his strength. 

3. Machinery enables us to 
use other motive powers besides 
our own strength. 

A horse without machinery can not lift 
a weight ; but he does it readily with the 
aid of the simple apparatus shown in Fig. 
84. Steam, applied directly to a boat, 
can not move it forward ; it is only with 
the help of machinery that it causes the 
wheel to revolve and thus produces mo- 
tion. Here, as in all other cases, the 




tion ? Show that perpetual motion is impossible. 198. If no additional power is gen- 
erated by machinery, why is it used ? What is the first advantage of using machine- 
ry? Give an example. If, with machinery, 10 men can do the work of 1,000, how 
long comparatively will it take them ? In some pieces of work, what difficulty pre- 



STRENGTH OF MATERIALS. 91 

power is not created by the machinery, but merely transmitted in a way 
to make it effective. 

Strength of Materials. 

199. There is a limit to the power of all machinery; 
and this limit is the strength of the materials of which it is 
made. Machines that work well in small models sometimes 
utterly fail when made of full size, because, when the resist- 
ance is increased and their own weight is added, no mate- 
rial can be found strong enough to stand the strain. 

Nature, also, recognizes this limit of size. Animals, after attaining a cer- 
tain age, cease to grow. If they kept on growing, they would soon reach 
such a size and weight that they could not move. If there were an animal 
much larger than the elephant, it would stagger under its own weight, unless 
its bones and muscles were thicker and firmer than any with which we are 
now acquainted. Fish, on the contrary, being supported by the water, move 
freely, no matter how heavy they may be. "Whales have been found over 50 
feet long and weighing TO tons — a monstrous size and weight, which no land 
animal could support. 

200. To determine how great a strain given materials 
will bear, and how they may be put together with the 
greatest advantage, becomes an important question in Prac- 
tical Mechanics. The relative strength of different sub- 
stances has been treated of under the head of Tenacity, on 
page 23. The following geueral principles relating to rods, 
beams, &c, should be remembered. 

1. Rods and beams of the same material and uniform 
size throughout, resist forces tending to break them in the 
direction of their length, with different degrees of strength, 
according to the areas of their ends. 

Let there be two rods of equal length ; if the areas of their ends are re- 
spectively 6 and 3 square inches, the one will bear twice as great a weight 

sents itself? What is the second advantage of using machinery? How is this exem- 
plified in the case of the farmer ? How, in the case of the porter? What is the thir.l 
advantage gained hy using machinery ? Illustrate this in the case of a horse. In tte 
case of steam. In hoth of these cases, what does the machinery merely do ? 199. What 
limit is there to the power of all machinery? Why do machines often fail, though 
6mall models of them work well ? Show how nature recognizes a limit of size. How 
is it that fish can move, though much larger and heavier than land animals ? 200. What 
important question is presented in Practical Mechanics ? What is the first principle 
laid down respecting rods and beams ? Give an example. When a rod is very long, 



92 MECHANICS. 

without breaking as the other. This law applies, no matter what the shape 
of the rods may be. 

2. When a very long rod is suspended vertically, its 
upper part, having to support more of the weight of the 
rod than any other, is the most liable to break. 

3. The strength of a horizontal beam supported at each 
end diminishes as the square of its length increases. 

If two beams thus placed are respectively 6 feet and 3 feet long, the 
strength of the shorter will be to that of the longer as the square of 6 to th« 
square of 3, — that is, as 36 to 9, or 4 to 1. 

4. A horizontal beam supported at each end, is most 
easily broken by pressure or a suspended weight in the 
middle, and increases in strength as either end is ap- 
proached. If, therefore, a beam of uniform strength is re- 
quired, it should gradually taper from the middle towards 
the ends. 

5. A given quantity of material has more strength when 
disposed in the form of a hollow cylinder than in any other 
form that can be given it. Nature constantly uses hollow 
cylinders in the animal creation, as in bones and the tubes 
of feathers; and the artisan, imitating nature, employs it 
in many cases where strength and lightness are to be com- 
bined. 

EXAMPLES FOR PRACTICE. 

1. (See §§ 183, 184.) "What is the horse-power of a steam-engine that can do 

1,650,000 units of work in a minute? 

2. What is the horse-power of an engine that can raise 2,376 pounds 1,000 

feet in a minute ? 

3. What is the horse-power of an engine that can raise 1,000 pounds 2,376 

feet in a minute ? 

4. A fire-engine can throw 220 pounds of water to a height of 75 feet every 

minute ; what is its horse-power ? 

5. A cubic foot of water weighs 62V2 pounds. How many horse-powers are 

required to raise 200 cubic feet of water every minute from a mine 132 
feet deep ? 

■(That part of it is most likely to break ? What law is given respecting the strength of 
« horizontal beam supported at each end ? Give an example. In what part is a hor- 
izontal beam supported at each end most easily broken by pressure ? What shape 
gives a beam uniform strength ? In what form must a given quantity of material be 
disposed, to have the most strength ? 



EXAMPLES FOR PRACTICE. 93 

6. A locomotive draws a train of cars, the resistance of which (caused by 

friction, &c.) is equivalent to raising 1,000 pounds, 15 miles an hour; 
what is its horse-power ? 
[Find how many feet the locomotive draws the train in a minute, and then 
proceed as be/ore.] 

7. How many pounds can an engine of 10 horse-powers raise in an hour from 

a mine 100 feet deep ? 

8. A certain man has strength equivalent to Vs of one horse-power; how 

many pounds can he draw up in a minute from a pit 25 feet deep ? 

9. (See § 189, Fourth Law.) If the friction of a train of cars weighing 50 tons, 

on a level railroad, be equivalent to a weight of 500 pounds, what will be 
the friction of a train weighing 25 tons ? of one weighing 100 tons ? of 
one weighing 60 tons ? 

10. (See §§195, 196.) C can just draw 75 pounds of coal a minute out of a 
mine. With the aid of a system of pulleys, he can raise 225 pounds at a 
time ; the friction being equivalent to 75 pounds, how many minutes 
will he be in raising the load ? — Ans. 4 min. 

[In practical questions of this hind, the friction must be added to the nsist- 
ance.] 

11. With a certain machine, one man can do as much as eight men without 
the machine. Allowing the friction of the machine to be equal to 
one-fourth of the resistance, how much longer will he be in doing a 
certain amount of work than they ?— Ans. 10 times. 

12. (See § 200.) [The area of a rectangular surf ace is found by multiplying 
its length by its breadth ; that of a triangle, by multiplying half its base 
by its perpendicular height.'] Which will support the greater weight 
without breaking, a joist whose section is 4 inches long by 5 broad, or one 
of the same kind of wood, 3 inches by 8 ? 

13. Which, when suspended, will bear the greater weight without breaking, 
a square rod of iron whose end is 3 inches by 3, or a rod whose cross sec- 
tion is a triangle with a base of 6 inches and a perpendicular height of 2 ? 

14. Two rods of copper, of equal length and uniform thickness, have ends re- 
spectively 4 inches by 2, and 17 inches by half an inch. Which, when 
suspended, will support the greater weight? 

15. Two horizontal beams of the same material, breadth, and thickness, sup- 
ported at both ends, are respectively 2 and 14 feet long. Which is the 
Stronger of the two, and how many times ? 



94 MECHANICS. 



CHAPTER VIII. 

MECHANICS (CONTINUED). 

THE MECHANICAL POWERS. 

Numerous and varied as machines are, they are all 
combinations of six Simple Mechanical Powers, known as 
the Le'-ver, the Wheel and Axle, the Pulley, the Inclined 
Plane, the Wedge, and the Screw. These we shall con- 
sider in turn. 

The I-ever. 

201. A Lever is an inflexible bar, capable of being moved 
about a fixed point, called the Fulcrum . 

The lever is the simplest of the mechanical powers. Its properties were 
known as far back as the time of Aristotle, 350 years b. c. Archimedes, a 
hundred years later, was the first to explain them full y. 

202. Kinds of Lever. — In the lever tiiree things are to 
be considered ; the fulcrum, or point of support, the weight, 
and the power. Two of these are at the ends of the bar, 
while the other is at some point between them. According 
to their relative position, we have three kinds of levers : — 

Fig. 85, A Lever of the First Kind is 

one in wh ; ch the fulcrum is be- 
tween the power and the weight ; 
as in Fig. 85, where F represents 
the fulcrum, J' the power, and W 
the weight. 

A Lever of the Second Kind is one in which the weight 
is between the power and the fulcrum ; as in Fig. 86. 

Of what are all machines combinations ? Name the six Simple Mechanical Pow 
ers. 201. What is a Lever? How does the lever compare with the other mechan- 
ical powers ? How long ago was it known ? 202. In the lever, how many things are 
to be considered? According to their relative position, how many kinds of levers 
are there ? What is a Lever of the First Kind ? What is a Lever of the Second Kind ? 




THE LEVER. 95 

Fig. 86. Fi & 87- 






1 



A Lever of the Third Kind is one in which the power is 
between the weight and the fulcrum ; as in Fig. 87. 

203. Law of the Levee. — The same law applies to 
all levers, whether of the first, second, or third kind ; viz., 
Intensity of force is gained, and time is lost, in proportion 
as the distance between the power and the fulcrum exceeds 
the distance between the weight and the fulcrum. 

204. Levers of the First Kind. — -In levers of the first 
kind, the relative position of the three important points is 

POWER FULCRUM "WEIGHT OR WEIGHT FULCRUM POWER. 

Fig. 88 shows one of the common- Fig. 88. 

est forms of this kind of lever, — the C^^^\ 

crow-har. The power is applied at the - /p 5 \ 

handle. The weight is at the other 
end, and consists of something to he 
moved. The fulcrum is a stone on 
which the crow-bar rests. Using an in- 
strument in this way is called prying. the crow-bar. 

According to the law of the lever, § 203, the nearer the 
fulcrum is to the weight the greater the power gained, and 
consequently the greater the space that P will have to 
pass through in moving W a given distance. 

Thus, in Fig. 88, if the distance from P to F be five times that from 
W to F, a pressure of 10 pounds at P will just counterbalance a weight of 
50 pounds at "W, or move any thing under 50 pounds ; while, for every 
inch that W is moved upward, P will have to move 5 inches downward. 

The distance through which the power must pass, to move a weight vast- 
ly greater than itself, becomes an important matter in practical applications 
-of-the. lever. _"When Archimedes saw the immense power that could be ex- 

What is a Lever of the Third Kind? 203. What is the eeneral law of the lever? 
204. In levers of the first kind, what is the relative position of the three important 
points? Give a familiar example of a lever of the first kind. Show how the law 
applies in levers of the first kind. What is sometimes an important matter in prac- 




96 



MECHANICS. 



erted with this instrument, he declared that with a place to stand on he could 
move the earth itself. He did not say how far he would have to travel to do 
this, in consequence of the great disproportion between his strength and the 
earth's bulk. Allowing that he had a place to stand on and a lever strong 
enough, and could pull its long arm with a force of 30 pounds through two 
miles every hour, it would have taken him, working ten hours a day, over 
one hundred thousand millions of years to move the earth a single inch ! 

205. The Balance. — When bodies of equal weight are 

supported by the arms of a lever, they will balance each 

Fig. 89. other when placed at equal distances 

# ^ • from the fulcrum, as in Fig. 89. They 

r are then said to be in equilibrium. 

Fig. 90. On this principle the com- 

mon Balance, represented in 
Fig. 90, is constructed. A 
beam is poised on the top of 
a pillar, so as to be exactly 
horizontal. From each end 
of the beam, at equal dis- 
tances from the fulcrum, a 
pan is suspended by means 
of cords. The object to be 
weighed is placed in one of 
these pans, and the weights 
in the other. 

When great accuracy is 
required, the beam is bal- 
anced on a steel knife-edge; 
the friction being thus les- 
sened, it turns more easily. A balance capable of weighing ten pounds has 
been made so sensitive as to turn with the thousandth part of a grain. 

206. The balance weighs correctly only when the arms of the beam are 
exactly equal. Hence dishonest tradesmen sometimes defraud those with 
whom they deal by throwing the fulcrum a little nearer one end of the beam 
than the other. When buying, they place the commodity to be weighed in 
the scale attached to the short arm: and, when selling, in the other, thus 
making double gains. To prove a balance, weigh an article first in one scale 
and then in the other ; if there is any difference in the weight, the balance is 
not true. 




THE BALANCE. 



tical applications of the lever? Show this in the supposed case of Archimedes. 
205. When are two bodies of equal weight, supported by the arms of a lever, said to 
be in equilibrium? What is constructed on this principle? Describe the Balance. 
When great accuracy is required, how is the beam balanced ? How sensitive has a 
balance been made ? 206. When does the balance weigh correctly ? How do dishon- 



THE STEELY AED. 97 

The true weight of a body may be determined, with a false balance, 
J>y placing it in either scale, balancing it with shot or sand, and then remov- 
ing the body and replacing it with weights till equilibrium is established. 
This is called double weighing. It must give the true weight ; for whatever 
error is made in the first weighing is corrected in the second. 

207. The Steelyard. — When bodies of unequal weight 
are supported by the arms of a lever, they will balance each 
other whenever the weight of the one multiplied into its 
instance from the fulcrum, is equal to the weight of the 
other multiplied into its distance from the fulcrum. 

In Fig. 91, let the distance WF be Fig. 91. 

one inch and PF three inches. The W F P 

weight of the one body, 30 pounds, mul- JP ^ 1 

tiplied into its distance from the fulcrum, Uq\ [10] 
1, gives 30 ; the weight of the other, 10 

pounds, multiplied into its distance from the fulcrum, 3, gives 30. These 
products being equal, the bodies will balance each other. 

208. On this principle the Steelyard is constructed. 
The Steelyard is a kind of balance, which, though not so 
sensitive as the one described above, answers very well for 
heavy bodies, and is conveniently carried, as it requires but 
a single weight, and may be held in the hand or suspended 
anywhere. 

Fig. 92 represents the steelyard. /«* Fig. 92. 

It is a lever of unequal arms ; from .^iitglhraifllMfr , , , , f, 
the shorter of which the article to 
be weighed is suspended, either di- 
rectly or in a scale-pan, while a con- 
stant weight is moved on the longer 
arm from notch to notch till equilib- 
rium is established. The number the steelyaed. 
at the notch on which the weight 

then rests, shows the required weight in pounds. Thus, 15 pounds is the 
weight of the sugar-loaf in the Figure. The proper distances for the notches 
are found in the first place by experiments with known weights in the scale- 
pan. 

To enable the steelyard to weigh still heavier objects without increasing 

*st tradesmen sometimes defraud those with whom they deal? How may a balance 
be proved ? How may the true weight of a body be determined with a false balance ? 
What is this process called? 207. When will bodies of unequal weight supported by 
the arms of a lever be in equilibrium? Illustrate this with Fig. 91. 208. What is 
constructed on this principle ? Describe the Steelyard, and the mode of weighing 
with it. How are the proper distances for the notches found in the first place ? Witk 

7 




98 MECHANICS. 

the length of its beam, it is often provided with an additional hook, hanging 
in an opposite direction from the other hook and nearer the point from which 
the article to be weighed is suspended. When the instrument is supported 
by this hook, a new fulcrum is formed, and the weight is shown by a new 
row of notches adapted to it. The greater the difference of length between 
the arms of a steelyard, the greater the number of pounds that it can weigh. 

209. When more than two bodies are supported on the 
arms of a lever, if the weight of each be multiplied by its 
distance from the fulcrum, the lever will be in equilibrium 
(that is, the bodies will balance each other) when the sums 
of the products on the two sides of the fulcrum are equal. 

Thus, in Fig. 93 equilibrium 
is maintained, because the prod- 
ucts of the weights on one side 
into their distances, added to- 
gether, equal the sum of the 
products on the other : — 

weights distances 

2X1 = 2 
6 X 3 = 18 

Sum of products, 20 Sum of products, 20 

210. Practical Applications. — Familiar examples of 
levers of the first kind are found in the scissors and pincers ; 
the rivet connecting the two parts being the fulcrum, the 
fingers the power, and the thing to be cut or grasped the 
weight. A poker introduced between the bars of a grate 
and allowed to rest on one of them, that purchase may be 
obtained for stirring the fire, is a lever of the first kind. So 
is the handle of a common pump. 

Fig. 94 When children teeter on a board 

balanced on a wooden horse, they use 
a lever of the first kind. According 
to the principles of the lever, if one is 
heavier than the other, to preserve the 
balance, he must sit nearer the fulcrum, as shown in Fig. 94. 

what are some steelyards provided, and for what purpose ? What steelyards weigh 
the greatest number of pounds ? 209. If more than two bodies are supported on the 
arms of a lever, when will they balance each other? Apply this principle in Fig. 93. 
210. Give some familiar examples of levers of the first kind. When children teeter 
on a board, what kind of lever do they use ? If one is heavier than the other, where 




£ 



BENT AND COMPOUND LEVERS. 



Fig. 95. 



211. Bent Levers. — Sometimes the arms of a lever are 
bent, instead of straight. In that case the same principles 
hold good, only that the arms of the lever are estimated, 
not by their actual length, but by the perpendicular dis- 
tance from the fulcrum to the line of direction in which the 
power and weight respectively act. 

As an illustration of bent levers of the first kind, 
we may take the truck used for moving heavy arti- 
cles, represented in Fig. 95. The axis on which the 
wheels turn represents the fulcrum ; the weight is ap- 
plied at W, and the power at P. The clawed side of 
a hammer, used in drawing out nails, is also a bent 
lever. The fixed point on which the head of the ham- 
mer rests is the fulcrum ; the friction of the nail is 
the weight ; and the power is applied at the extrem- 
ity of the handle. 

212. Compound Levers. — Simple le- 
vers of the first kind may be combined 
into Compound Levers. 

213. In compound levers, equilibrium is established when 
the power, multiplied by the first arms of all the levers, is 
equal to the weight multiplied by the last arms of all the 
levers. 

Fig. 96. 




THE HAND-TKUCK. 




A COMPOUND LEVEE. 

Thus, in Fig. 96, which represents a compound lever formed of three sim- 
ple ones, let the long arm of each lever be three times the length of its short 
arm ; then 1 pound at P will balance 27 pounds at W, because 
1 pound X3X3X3 = 27 pounds X 1 X 1 X 1. 

214. Levees of the Second Kind. — In levers of the 
Second kind, the relative position is 

must he sit to preserve the balance ? 211. What is meant by a bent lever ? How are 
the arms of a bent lever estimated ? Give some familiar examples of bent levers. 
212. How may simple levers of the first kind be combined ? 213. When is equilib- 
rium established in a compound lever ? Illustrate this with Fig. 96. 214. In levers 



100 



MECHANICS. 



POWER WEIGHT FULCRUM OR FULCRUM WEIGHT POWER. 



Fig. 97. 




Fig. 97 shows how the crow-bar may 
be used as a lever of the second kind. 
The power is applied at the handle ; the 
fulcrum is at the other end, and the 
weight to be moved is between them. 

215. Levers of the second 
kind, like those of the first, are 
used to gain power at the ex- 
pense of time. According to 
the law of the lever, stated in 
§203, the nearer the weight is to* the fulcrum the more 
power is gained, and consequently the greater is the space 
that P will have to pass through in moving W a given dis- 
tance. 

Thus, in Fig. 97, if the distance P F be five times as 
great as W F, a pressure of 10 pounds at P will counter- 
balance a weight of 50 pounds at W, and move any thing 
under 50 pounds ; while, for every inch that W is moved, 
P will have to move five inches in the same direction. 

216. Practical Applications. — The 
common chipping-knife, used by apothe- 
caries, and represented in Fig. 98, is a 
familiar illustration of levers of the sec- 
ond kind. The knife is fastened at one 
end, F, which thus becomes the fulcrum ; the hand is ap- 
plied, as the power, at the other end, P ; and the substance 
to be cut is the resistance, or weight, between them. Nut- 
crackers and lemon-squeezers work on the same principle, 
and are levers of the second kind. 

A door turned on its hinges, and an oar used in rowing, 
are also examples of this kind of lever. In the former case, 
the hinge is the fulcrum ; the hand applied at the knob is 
the power; and the weight of the door, which may be re- 



Fig. 98. 




THE CIIIPPING-KNIFE. 



of the second kind, what is the relative position of the three important points ? How 
may the crow-bar be used as a lever of the second kind ? 215. How does the general 
iaw of the lever apply to levers of the second kind ? Give an example. 216. What fa- 
miliar articles are levers of the second kind ? Show how a door turned on its hinges is 



LEVERS OF THE THIRD KIND. 101 

garded as concentrated in its centre of gravity somewhere 
between the two, is the resistance. In the latter case, the 
point at which the oar enters the water is the fulcrum ; the 
rower's hand is the power ; and the weight of the boat, 
acting at the row-lock, is the resistance. According to the 
law laid down in § 215, the further from the row-lock we 
grasp the oar, the more easily we overcome the resistance 
and produce motion. 

217. Two persons carrying a weight suspended from a stick between them, 
use a double lever of tbe second kind. Power is applied at each end, and 
each end in turn becomes the fulcrum to the other, the weight resting on 
some intermediate point. The relation of the power at one end to the weight 
is governed by the same law as that of the power at the other end; and there- 
fore the weight, to be divided equally, must be suspended from the middle of 
the stick. If it is not so suspended, the man who is nearer the weight car- 
ries more than the other in proportion as he is nearer. 

Thus, let a 12-pound weight, W, be suspended Fig. 99. 

from a bar three feet long, at a distance of one foot -A 

from A and two feet from B. Then A will carry 
two-thirds of the weight, and B one-third. On 
this principle, when it is desired that one of the horses harnessed to a car- 
riage should draw more than the other, it is necessary only to make the arm 
of the whiffle-tree to which he is attached proportionally shorter. 

Fig. 100 shows how a weight may be equally q Fig. loo. 

distributed between three persons. B, being o/- M. g 

twice as far from E as D is, bears one-third of the A/ 1 

weight, W ; while A and C, at the extremities ^ 

of the equal-armed lever ADC, bear equal portions of the remaining two 

thirds, or one-third each. 

218. Levers of the Third Kind. — In le- 
vers of the third kind, the relative position is 

FULCRUM POWER WEIGHT OR WEIGHT POWER FULCRUM. 

The forceps, represented in Fig. 101, is a lever of the 
third kind. The two sides unite at one end to form the ful- 
crum ; the article to be grasped is the weight ; and the fin- 
gers, applied between the two, constitute the power. 

219. Levers of the third kind, unlike those 
before described, involve a mechanical disad- 

a lever of the second kind. Show how an oar acts as a lever of the second kind. 
217. When two persons carry a weight suspended from a stick between them, what 
kind of a lever do they use ? Where is the fulcrum ? To be equally divided, where 
must the weight be suspended? If the weight does not hang from the middle of the 




102 MECHANICS. 

vantage ; that is, to produce equilibrium, the power must 
always be greater than the weight. 

Law. — According to the law of the lever, § 203, inten- 
sity of force is lost, and time is gained, in proportion as 
the distance from the weight to the fulcrum exceeds the 
distance from the power to the fulcrum. 

Thus, in Fig. 101, if FW be three times as great as FP, it will require a 
power of three pounds at P to counterbalance a resistance of one pound at 
W. Levers of this class, therefore, are never used when great power is re- 
quired, but only when a slight resistance is to be overcome with great ra- 
pidity. 

220. Practical Applications. — The sugar-tongs, which 
resembles in shape the forceps above described, is a familiar 
example of the third kind of lever. So is the fire-tongs ; 
and hence the difficulty of raising heavy pieces of coal with 
this instrument, particularly when the hand is applied near 
the rivet or fulcrum. 

The sheep-shears is another lever of the third kind, admirably adapted to 
the work it performs ; because the wool, being flexible, has to be cut rapidly, 
while it does not require any great degree of force. 

A door becomes a lever of the third kind when one attempts to move it by 
pushing at the edge near the hinges. The mechanical disadvantage is shown 
by the great strength required to move it when the power is there applied. 
So, when a painter attempts to raise a ladder lying on the ground with its 
bottom against a wall, by lifting the top and walking under it grasping round 
after round in succession, he experiences great difficulty as he approaches 
the bottom, because the ladder, when he passes its centre of gravity, becomes 
a lever of the third kind. 

Fig. 102. ^0 Nature uses levers of the 

third kind in the bones of 

animals. The fore-arm of a 

man, represented in Fig. 

human arm and hand. 1 02, will serve as an example 

9tick, which man will carry the more ? Illustrate this with Fig. 99. How may one 
of the horses harnessed to a carriage he made to draw more than the other ? How 
may a weight he equally distributed between three persons ? 218. In levers of the- 
third kind, what is the position of the three important points ? What instrument is 
an example of the third kind of levers ? 219. To produce equilibrium in the third kind 
of levers, what is necessary ? State the law for levers of the third kind. Illustrate 
this with Fig. 101. 220. What common articles are levers of the third kind ? What 
is said of the sheep-shears ? When does a door become a lever of the third kind ? 




THE WHEEL AND AXLE. 



103 



The fulcrum, F, is at the elbow-joint ; the biceps muscle, descending from 
the npper part of the arm and inserted near the elbow at P, operates as the 
power ; while the weight, W, rests on the hand. If the distance F W be 15 
times as great as F P, it will take a power of 15 pounds at P to counterbal- 
ance one pound at W ; and when the arm is extended, the disadvantage is 
. still greater, in consequence of the muscle's not acting perpendicularly to the 
bone, but obliquely. 

This accounts for the difficulty of holding out a heavy weight at arm's 
ength. In proportion as power is lost, however, quickness of motion is 
gained ; a very slight contraction of the muscle moves the hand through a 
comparatively large space with great rapidity. Here, as in all the works of 
creation, the wisdom of Providence is shown in exactly adapting the part to 
the purpose for which it is designed. With so many external agents at his 
command, man does not need any great strength of his own ; quickness of 
motion is much more necessary to him, and this the structure of his arm 
ensures. 

The Wheel and Axle. 

221. The Wheel and Axle is the second of the simple 
mechanical powers. It consists of a Wheel attached to a 
cylinder, or Axle, in such a way that when set in motion 
they revolve around the same axis. 

222. In the simplest form of the wheel and axle, the 
power is applied to a rope passmg round the wheel, while 
the weight is attached to another 
rope passing round the axle. 

This form of the machine is shown in Fig. 103. 
C D is a frame" ; B is the wheel ; A is the axle, 
attached to the frame at its extremities E and F 
by gudgeons, or iron pins, on which it turns. P 
is the power, and W is the weight. 

223. The wheel and axle is simply 
a revolving lever of the first kind. 
One application of the lever can not 
move a body any great distance ; but, 
by means of the wheel and axle, the 
action of the lever is continued unin- 




THE WHEEL AND AXLE. 



Under what circumstances does a ladder become a lever of the third kind ? In what 
ioes Nature use levers of the third kind ? Show, by Fig. 102, how the fore-arm is a 
lever, and point out the relation between power and weight. How is the wisdom of 
Providence shown, in making the arm such a lever ? 221. What is the second sim- 
ple mechanical power ? Of what does the Wheel and Axle consist ? 222. In the sim- 
plest form of this machine, how is the power applied, and how the weight ? Illus 



104 MECHANICS. 

terruptedly. This machine has therefore been called the 
perpetual or endless lever. 

224. The wheel and axle must turn round their common axis in the same 
time. In each revolution, a length of rope equal to the wheel's circumfer- 
ence is pulled down from the wheel, while only as much rope is wound round 
the axle as is equal to the axle's circumference. There is, therefore, a loss ■ 
of time, greater or less according as the circumference of the wheel exceeds 
that of the axle ; but, by the law of Mechanics already stated, there must be 
a corresponding gain of power. 

Viewing the wheel and axle as a lever of the first kind, we have the cir- 
cumference of the wheel for the long arm, and that of the axle for the short 
arm. If the diameters of the wheel and the axle are given instead of their 
circumferences, they may be taken for the two arms ; and so with the radii, 
if they are given. In practice, an allowance of 10 per cent, of the weight 
must be made for the stiffness of the ropes and the friction of the gudgeons. 
— From these principles is deduced the following law : — 

225. Law of the "Wheel and Axle. — With the wheel 
and axle, intensity of force is gained, and time is lost, in 
proportion as the circumference of the wheel exceeds that of 
the axle. 

Thus, in Fig. 103, if the circumference of the wheel B is five feet and that 
of the axle A one foot, a power of 40 pounds at P will counterbalance a weight 
of 200 pounds at W, and of course lift any thing under 200 pounds. 

226. Different Forms. — The wheel and axle is exten- 
sively used, and assumes a variety of forms. 

Fig. 104. Instead of having a rope attached to it, the 

wheel is often provided with projecting pins, a» 
shown in Fig. 104, to which the hand is directly 
applied. This form of the machine is used in 
the pilot-houses of steamboats for moving the 
rudder. In calculating the advantage in this 
case, instead of the circumference of the wheel 
we must take the circumference of the circle 
described by the point to which the hand is ap- 
plied. 

A still more common form, much used in drawing water from wells and 
loaded buckets from mines, is shown in Fig. 105. Instead of a wheel, we 



trate this with Fig. 103. 223. What has the wheel and axle been called, and why ? 
224. Explain the operation of the wheel and axle, and show how great the loss of time 
and gain of power will be. Viewing the wheel and axle as a lever, what is the long 
arm ? What is the short arm ? What, besides the circumference, may be taken as 
the arms of the lever ? What allowance must be made in practice "? 225. State the law 
if the wheel and axle. Illustrate this law with Fig. 103. 226. Describe the form oi 




CAPSTAN AND WINDLASS. 



105 



Fig. 105. 





have here a Winch, or handle, attached to the axle. 
In this case, to calculate the advantage gained, we 
must compare the circle described by the extrem- 
ity of the handle (shown in the Figure by a dotted 
line) with the circumference of the axle. 

Fig. 106. Fi S- 10(J shows a 

..— — .._ third form of the 

wheel and axle. 

Here the axle A is 

vertical, instead of horizontal. A bar insert- 
ed in its head, at the extremity of which the 
hand is applied, takes the place of the wheel. 
If the circumference of A is 3 feet and the 
circle described by P is 12 feet, a power of 1 
pound at P will counterbalance a weight of 4 
pounds at W. 

227. The Capstan. — The Capstan (see Fig. 107) is a fa- 
miliar example of this form of the wheel and axle. It is 
used by sailors for warping vessels up to a 
dock, raising anchors, &c. ; and consists of a 
massive piece of timber, round which a rope 
passes. This is surmounted by a circular head, 
perforated with holes, into which, when the in- 
strument is to be used, strong bars, called 
handspikes, are inserted. Several men may work at each 
handspike, pushing it before them as they walk round the 
capstan. The handspikes act on the principle of the lever. 
The longer they are, therefore, the more easily the men 
overcome the resistance, but the further they have to walk 
in doing it. 

228. The Windlass. — This is a similar form of the wheel 
and axle, used on shipboard for various purposes. 

The windlass is not vertical, like the capstan, but horizontal or parallel 
to the deck. It is a round piece of timber, supported at each end, and per- 
forated with rows of holes. Pushing against handspikes inserted in these 




THE CAPSTAN. 



the wheel and axle used in the pilot-houses of steamboats. In calculating the advan- 
tage in this case, what must we substitute for the circumference of the wheel? De- 
scribe the form of the machine used in drawing water from wells. How is the ad« 
vantage ascertained in this case? Describe a third form of the wheel and axle, 
exhibited in Fig. 106. 227. What machine is a familiar example of this third form? 
For what is the Capstan used ? Of what does it consist ? How is it worked ? How do 
thts handspikes act ? 228. What similar instrument is often substituted for the cap« 



106 



MECHANICS. 



holes, the boatmen turn the barrel of the windlass halfway over. It Is held 
there by a suitable apparatus, till the handspikes are removed and put in a 
new row of holes, when the process is repeated. The windlass acts on the 
same principle as the capstan, but is less convenient, on account of the man- 
ner in which the force is applied, and the necessity of removing the hand- 
spikes to new holes from time to time. 

229. Wheels enter largely into machinery. The modes 
of connecting them will be considered hereafter. 




THE PULLEY. 



The Pulley. 

230. The Pulley is the third of the simple mechanical 
Fig. 108. powers. It consists of a wheel with a 

grooved circumference, over which a rope 
passes, and an axis or pin, round which 
the wheel may be made to turn. The 
ends of the axis are fixed in a frame 
called a block. 

Fig. 108 gives a view of the pulley. A represents 
the block, B the axis, and C the wheel. Round the 
groove in the wheel passes a rope, at one end of 
which the power acts, while the weight is attached 
to the other. 

231. Kinds of Pulley. — Pulleys are of two kinds, — 
Fixed and Movable. 

232. Fixed Pulleys. — A Fixed Pulley is 
one that has a fixed block. 

Fig. 109 represents a fixed pulley. The block is at- 
tached to a projecting beam. P is the power, and W the 
weight. For every inch that P descends, W ascends the 
same distance. There is, therefore, no loss of time, and no 
gain in intensity of force. One pound at P will just coun- 
terbalance one pound at W. 

233. la this rule, as well as all the others pertaining 
to the Mechanical Powers, it must be remembered that 
friction is not taken into account. In the case of the pul- 
ley, in consequence of the stiffness of the rope and the 
friction of the pin, an allowance of 20 per cent, of the 
weight, and often more, must be made in practice. 



Fig. 109. 




FIXED PULLET. 



stan ? How does the Windlass differ from the capstan ? Of what does the windlass 
consist ? How is it worked ? What makes it less convenient than the capstan ? 
229. What is said of wheels ? 230. What is the third simple mechanical power ? Of 



FIXED PULLEYS. 



107 



Fig. 110. 




234. Though no power is gained with the fixed pulley, 
rt is frequently used to change the direction of motion. 
The sailor, instead of climbing the mast to hoist his sails, 
stands on deck, and by pulling on a rope attached to a 
pulley raises them with far less difficulty. With equal ad- 
vantage the builder uses a fixed pulley in raising huge 
blocks of stone or marble, and the porter in hoist- 
ing heavy boxes to the lofts of a warehouse. 

235. With two fixed pulleys, horizontal motion 
may be changed into vertical ; horses are thus en- 
abled to hoist weights, as shown in Fig. 84. 

236. Fig. 110 shows how a person may raise 
himself from the ground, or let himself down from 
a height, by means of a fixed pulley. In lofty 
buildings an apparatus of this kind is sometimes 
rigged near a window, to furnish means of escape 
in case of fire. 

237. Movable Pulleys. — A Movable Pulley 
is one that has a movable block. 

Fig. Ill represents a movable pulley. A is the wheel. 
One end of the rope is fastened to a support at D, while 
the power is applied to the other at P. 

238. To raise the weight a given distance with the 
movable pulley, the hand must be raised twice that dis- 
Fig. 112. tance. Time, therefore, being lost in the 

proportion of 2 to 1, the intensity of the 
force is doubled. A power of one pound 
at P will counterbalance two pounds at 
W, and raise anything under two pounds. 

239. A movable pulley is 

seldom used alone. It is gen- 

(r^r erally combined with a fixed pulley, as shown 

in Fig. 112. No additional power is thus 



Fig. Ill 





MOVABLE PULLEY. 



^hat does the Pulley consist ? What is the Block ? Point out the parts of the pub 
.ey in Fig. 103. 231. How many kinds of pulleys are there ? 232. What is a Fixed 
Pulley? Point out the parts in the Figure. What is the gain with this pulley? 
233. What allowance must he made for friction in the case of the pulley ? 234. If no 
power is gained by the use of the fixed pulley, why is it used? Give examples. 
935. How may horizontal motion be changed into vertical ? 236. What does Fig. 110 



108 



MECHANICS. 



Fi s- m gained ; on the contrary, there is a loss, the 
friction of two pulleys being double that of one. 
But this loss is more than counterbalanced b^ 
the greater convenience of pulling downward. 

240. When a high degree of force is required, several mov- 
able and fixed pulleys may be combined, as represented in 
Fig. 113. A and B are fixed pulleys ; C and D are movable 
ones, from the block of which the weight W is suspended. 
One end of the rope is attached to the lower extremity of the 
fixed block, F ; to the other end the power is applied, after the 
rope has passed in succession over each of the four pulleys. 

To move W an inch with this combination, each length of 
rope must be shortened an inch, and therefore P must move 
as many inches as there are lengths of rope. Since there are 
two lengths of rope for each movable pulley, we .may lay down 
the following law : — 

241. Law of Movable Pulleys. — With mov- 
able pulleys, a power will balance a weight as many times 
greater than itself as twice the number of movable pulleys 
employed. 

In Fig. 113, a power of 1 pound will balance a weight of 4 pounds. If 
three movable pulleys were used, 1 pound at P would balance 6 pounds at W ; 
if four were used, 8 pounds, &c. Friction, however, nullifies much of this 
gain. 

242. White's Pulley. — To lessen the friction, when a 
number of pulleys are required, the wheels are made to 
turn on the same axis. This is effected by having but one 
block for all the upper pulleys, and one for the lower ; 
grooves being cut in each, to take the place of separate 
wheels. The friction in each block is thus reduced to that 
of a single wheel. This system is called, from its inventor, 
White's Pulley. 

Fig. 114 gives a front and a side view of White's Pulley. A is the fixed 



show ? For what is an apparatus of this kind sometimes used ? 237. What is a Mov- 
able Pulley? Describe it with Fig. 111. 238. To move a weight a given distance 
with a movable pulley, how far must the power travel ? What, then, is the law of 
his machine' 239. With what is a movable pulley generally combined ? What is 
gained by this combination ? 240. Describe the combination of movable and fixed pul- 
leys represented in Fig. 113. 241. What is the law of movable pulleys? Apply this 
law in the case of the pulley represented in Fig. 113. By what is much of this gain 
nullified? 242. When a number of pulleys are required, how is the friction lessened? 



MOVABLE PULLEYS. 



109 



Fig. 114. 



block, with grooves of different sizes representing the separate wheels. B is 

the movable block, similarly prepared. A single rope is used, which is 

fastened at one end to the smallest fixed 

pulley, and acted on by the power at the 

other. Here again, if friction is left out of 

account, the power will counterbalance a 

weight as many times greater than itself as 

twice the number of movable pulleys. In 

Fig. 114 there are six movable pulleys ; 

consequently, with a pressure of 1 pound 

at P, equilibrium will be established when 

W is twice six, or 12, pounds. 

243. Fig. 115 shows another 

system of movable pulleys, each 

of which has a separate rope of 

its own attached at one end to a 

fixed support. 

Fig. 115. To raise the low- 

est pulley, A, and the 
weight suspended 
from it one inch, two 
inches of its rope 
must be pulled up. 

This is done by pull- white's pallet. 

ing up twice 2, or 4, inches of B's rope ; and this, in 
turn, by pulling up twice 4, or 8, inches of C's rope- 
P, therefore, must descend 8 inches, to raise W one inch. 
If there were four movable pulleys, P would have to de- 
scend 16 inches to raise W one inch ; if 5, 32 inches, and 
so on, — P's distance doubling for each new pulley add- 
ed. Hence, with this combination, the power balances a 
weight as many times greater than itself as 2 raised to thd 
denoted by the number of movable pulleys. 

244. The pulley is so cheap and conve' 
nient that it is much used in its simple forms. In com- 
plicated systems, more than half the advantage is lost by 
friction and the stiffness of the ropes ; and consequently 
such systems are used only when immense weights are to 
be raised. 





What pulley Is constructed on this principle? Describe White's Pulley. With 
White's Pulley, what is the gain ? 243. Describe the system of pulleys represented in 
Fig. 115. Explain its operation. What is the gain with this system ? 244. What is 



L10 



MECHANICS. 




The Inclined Plane. 

245. The Inclined Plane is the fourth of the simple me- 
chanical powers. It is a plane surface, inclined to the ho- 
rizon at any angle. Every road not perfectly level is an 
inclined plane. 

Fig. 116. A D, in Fig. 116, is an inclined plane, 

of which AC is the length, AB tho 
height, and B C the base. In theory, 
an inclined plane is perfectly smooth 
\ and hard. No such surface, however, 

\ c X, exists; and, therefore, in estimating 

the inclined plane. the advantage of this machine for prac- 

tical purposes, allowance must be made for friction, according to the irregu- 
larity or softness of the surface. 

246. When a body is moved over a horizontal surface, its weight is sup- 
ported, and the resistance of the air and friction are all that have to be over- 
come. When a body is lifted perpendicularly, there is no friction, but we 
must overcome the whole weight and the resistance of the air. When a body 
is drawn up an inclined plane, the resistance of the air, friction, and a por- 
tion of the weight must be overcome, — more or less of the weight being sup- 
ported, according to the inclination of the plane. It is, therefore, harder to 
move a body up an inclined plane than over a level surface, as we know 
by dragging a wagon up hill ; but it is easier than to lift it to the same 
height. 

247. Law. — When the power acts parallel to the inclined 
surface, intensity of force is gained, and time is lost, in 
proportion as the length of the plane exceeds its height. 

Thus, in Fig. 117, let the length of 

the plane A B be 12 feet, and its height 

4 feet ; then 1 pound at P will counter- 

- -ia ^--<: balance 3 pounds at W. 

'jhj| _ ^^^^'^^^^t^ With a given height, the longer th« 

plane the easier it is to raise an object 
upon it. Hence, on steep mountains, 
the road is not carried from the bottom 



Fig. 117. 




said of the pulley in its simple forms? What is said of complicated systems of pul- 
leys ? 245. What is the fourth simple mechanical power? What is the Inclined Plane? 
For what must allowance be made, in estimating the advantage of the inclined plane, 
andwny? 246. As regards the resistance" "to" be overcome, show the difference be- 
tween moving a body over a horizontal surface, lifting it, and drawing it up an in- 
clined plane. 247. What is the law of the inclined plane ? Illustrate this law with 
Fig. 117. How is the road up a steep mountain frequently made, and why ? How 



THE INCLINED PLANE. Ill 

directly to the top, but winds round the sides. Instinct teaches a horse this 
principle ; for, if left to himself in ascending a hill, he does not go straight 
up, but moves in a zigzag course from one side of the road to the other, thus 
taking more time, but making the ascent easier. 

248. Practical Applications. — When hogsheads or 
heavy boxes are to be raised into carts or pulled up a flight 
of stairs, the work is facilitated by laying long planks, or 
nkids, in such a way as to form an inclined plane. A piece 
of board is similarly placed, if a carriage or wheelbarrow 
has to be raised over a high curb-stone. — The marine rail- 
way, on which ships of immense weight are drawn out of 
the water, to be repaired, is one of the most useful appli- 
cations of this machine. 

249. The inclined plane was known to the ancients. It 
is supposed that the Egyptians used it in raising the huge 
blocks of stone employed in the construction of their pyra- 
mids. 

250. Law of Bodies rolling down an Inclined Plane. — 
When bodies are allowed to roll down an inclined plane, 
they have a uniformly accelerated motion, and attain the 
same velocity by the time they reach the bottom that they 
would have if dropped perpendicularly from the starting 
point. 

A ball dropped from a height of 6473 feet, when it strikes the ground, has 
a velocity of 6473 feet in a second. If it were allowed to roll from the same 
height, down an inclined surface a mile long, perfectly smooth and hard, it 
would have the same velocity on reaching the bottom. The shorter the plane, 
the less time it would take for its descent and the sooner it would acquire the 
velocity in question. 

251. When the perpendicular height is considerable, objects rolling or 
sliding down an inclined plane acquire, near the bottom, a prodigious veloci- 
ty. A remarkable instance of this was exhibited at a slide near Lake Lucerne, 
Switzerland, down which fir-trees were allowed to descend, from the top of a 
mountain. The slide was about eight miles long ; and, though the descent was 
but 300 feet to a mile and the road was often circuitous, the trees went tearing 
along with frightful speed, performing the whole distance in six minutes. 



ioes ahorse ascending a hill display his instinct? 248. In what familiar cases is the 
inclined plane used ? What is one of the most useful applications of this machine ? 

249. By whom is the inclined plane thought to have been used in ancient times? 

250. What is the law of bodies rolling down an inclined plane ? Illustrate this. 
851. "When do boiies sliding down an inclined plane acquire a prodigious velocity ? 



112 MECHANICS. 

The Wedge. 

252. The Wedge is the fifth of the simple mechanical 
powers. It appears in two forms, according to the use for 
which it is designed. 

253. First Kind of Wedge. — In its first form, the 
wedge is simply a solid and movable inclined plane, used 
for raising great weights a short distance. The power 
(acting parallel to the base, instead of the inclined surface) 
counterbalances a weight as many times greater than itself 
as the height of the wedge is contained in its base. 

-p i<T ug Fig- US shows how the wedge may be used 

for raising weights. WD is a pillar, so fixed 
that it can not move, except perpendicularly 
upward. A B is a wedge resting on its base. 
The sharp edge being brought near the ex- 
tremity of the pillar, power is applied to the 
side B C. W must rise, as it can not move in 
any other direction. By driving the wedge 
under to C, the pillar is raised the distance 
BC. 
Fig. 119. A more common mode of raising bodies 

with this machine is shown in Fig. 119. A and 
B are similar wedges. Simultaneous blows 
are given them at A and B in opposite direc- 
tions with heavy mallets, and the weight W is 
slowly raised. The same power must be ap- 
plied to each as if it acted alone. Twice as 
much power, therefore, is required as when 
but one wedge is used, but the weight is raised twice as high in a given time. 
254. Thus applied, the wedge is an efficient and useful machine. It raises 
immense weights, though to no great distance. With its aid, ships are 
brought up on the dry dock, and houses thrown out of line by the sinking oi 
their foundations are restored to the perpendicular. Wedges are also used 
in extracting oil from seeds. The seeds are placed between immovable tim- 
bers, in bags that allow the liquid, as it is pressed out, to ooze through. Be- 
tween the bags are then inserted wedges, which are gradually driven in. So 
intense is their pressure that every particle of oil is extracted, and the seeds, 
when taken out, are found mashed together, into a dense solid mass. 

What instance of this is mentioned ? 252. What is the fifth mechanical power ? Ii. 
now many forms does the wedge appear? 253. Describe the first kind of wedge. 
For what is it used ? What law does it follow ? Describe the operation of this sort 
of wedge with Fig. 118. What is the more common mode of raising bodies with this 






THE WEDGE. 113 

255. Familiar Applications.— Chisels and other tools sloped, or chamfered, 
as it is called, on only one side, are familiar examples of this sort of wedge. 
The longer the chamfered part in proportion to its thickness, the more easily 
the chisel overcomes the resistance of the wood into which it is driven. 

256. Second Kind of Wedge. — The second kind of 
wedge (see Fig. 120) has the shape of two inclined planes 
united at their bases. It is used for Fig. 120. 
splitting timber and rending rocks 
in quarries. 

The resistance overcome by the wedge, when 
thus used, is the cohesion of the substance to be 
split. As long as the wedge is merely pressed the wedge. 

against this substance, little or nothing is effected ; but, when driven in with 
blows, it becomes a highly useful instrument. When once forced in, it is 
prevented from receding by the friction of the wood against its sides. Thus 
every blow begins to act where the preceding blow left off acting. 

257. Advantage gained. — The exact advantage gained 
by this sort of wedge when driven in by blows, can not be 
computed. The percussion gives such a shock to the par- 
ticles that they open a little in advance of the wedge, as 
shown in Fig. 120, and readily allow it to enter. 

The only law we can lay down is this : — With a given 
thickness, the longer a wedge is, the more easily it pene- 
trates. 

258. Familiar Applications. — Knife and razor blades, 
the heads of axes and hatchets, nails, and all cutting in- 
struments chamfered on both sides, are examples of this 
kind of wedge. Pins and needles may be looked upon as 
wedges with an infinite number of sides. In all these, the 
longer the instrument in proportion to its thickness, the 
greater the advantage gained. 

259. In seeking to increase the advantage of the wedge by lengthening it, 
care must be taken not to make it too long. A slender tool will answer for 

machine ? What is said of the power in this case ? 254. For what is the first kind of 
wedge used? Describe the mode of extracting juices from seeds. 255. What tools 
are examples of this kind of wedge ? On what does the ease with which they over- 
come the resistance depend ? 256. Describe the second kind of wedge. For what is 
It used ? What sort of power must he applied to the wedge, when thus used ? What 
prevents the wedge from receding? 257. What is said of the advantage gained by 
the wedge? What is the only law that can be laid down for this machine ? 258. Men- 
tion some familiar examples of the second kind of wedge. 259. In the case of the 
8 



114 MECHANICS.. 

soft substances, but not tor hard. A carpenter's chisel, for instance, whose 
chamfered edges make an angle of 30 degrees, would soon break if used on 
iron. When this metal is to be cut, the edges should make an angle of 60 
degrees, and for copper at least 80. 

The Screw. 

260. The screw is the sixth and last of the simple me- 
chanical powers. It consists of a cylinder with a spiral 
Fig 121 ridge and groove winding alternately round it in 
A parallel curves. The portions of the ridge passing 
successively from one side of the cylinder to the 
other are called the Threads of the screw. 

Fig. 121 represents a screw. If we could unwind the threads 
from the cylinder, commencing at the end A, we should have a 
continuous wedge. The back of this wedge is applied to the cyl- 
inder ; and on its thickness depends the distance between the 
threads of the screw. 

261. Kinds or Sckew. — Screws are of two 
kinds : — 

1. The Exterior or Convex Screw, represented in Fig. 

121, in which the ridge and groove are on the out- 
side of the cylinder. 

2. The Interior or Concave Screw, in which the ridge 

and groove are on what may be regarded as the 
inside surface of a cylinder. 

These two forms are used together, and are generally 
called the Screw and the Nut. Every screw must have a 
nut grooved in such a way as to receive its ridge. 

262. Advantage gained. — The power is applied at the 
head of the screw. The resistance is to be overcome by 
pressure produced at its other end. Every time the screw 
is turned once round in the nut, it advances as far as the 
distance between two of its threads, and compresses to that 



wedge, what must be avoided ? What difference is there between a carpenter's chisel 
and one suitable for iron and copper? 260. What is the sixth mechanical power? 
Of what does the Screw consist ? What is meant by the Threads of the screw ? If 
we could unwind the threads from the cylinder, what would they form? 2(51. How 
many kinds of screws are there ? Name and describe each. How are these two forms 
of the screw used, and what are they generally called ? 262. With the screw, how 



THE SCREW. 



115 




THE SCREW AND NUT. 



Fig. 123. 



extent any fixed object against which it is directed. With 
the screw, therefore, the power produces a pressure as many 
times greater than itself, as the circumference of the head 
is greater than the distance between the threads. 

Here, again, however, friction lessens the ef- 
fect ; and, to gain greater power, a lever is gen- 
erally combined with the screw. The mode of 
doing this is shown in Fig. 122, in which S is 
the screw and L the lever. 

In calculating the advantage in this case, in- 
stead of the circumference of the head take the 
circle described by the point of the lever at which 
the hand is applied. In Fig. 122, let the distance 
between the threads be 1 inch and the dotted 
circle 100 inches; then (friction being left out 
of account) a power of 1 pound at the extremity 
of the lever will produce a pressure of 100*pouuds 
at the lower end of the screw. 

263. Book-binder's Press. 
— The Book-binder's Press, rep- 
resented in Fig. 123, exhibits 
one of the most useful and con- 
venient modes of applying the 
screw. 

S S is a screw, playing in a station- 
ary nut in the head of the press. At- 
tached to the screw near its bottom are 
two bars at right angles to each other, 
at the extremities of which the hand is 
applied when the press is to be worked. 
Still greater leverage is obtained by ap- 
plying the power at the end of a bar, P, 
introduced successively into holes in the 
extremities of the cross-pieces, as in 
working the windlass. A fall or platen, 
B B, is attached to the screw, in such a 
way that it does not turn as the screw revolves, but must rise or descend with 
it. Between this fall and the bed of the press, D, the books to be pressed are 




BOOK-BINDERS PRESS. 



great a pressure does the power produce ? In practice, what lessens the effect ? How 
is greater power obtained ? When a lever is combined with the screw, how may we 
find the advantage gained? Illustrate this with Fig. 122. 263. What machine ex- 
hibits a useful application of the screw? Describe the boot -binder's press. How is 



116 



MECHANICS. 



placed. Here again, to obtain the advantage, divide the circumference of the 
circle described by P, by the distance between the threads. 

264. Screws, applied in this or some similar way, are 
extensively used when a great and continued pressure is 
required within a small space. Cotton is compressed into 
bales, juices are extracted from fruit, coins are stamped, 
and houses are raised from their foundations, with the aid 
of the screw. 

265. Hunter's Screw. — When intense pressure is re- 
quired, the threads of the screw have to be so close to- 
gether that they are necessarily thin and liable to break, 
To prevent this, an ingenious contrivance, called after its 
inventor Hunter's Screw, is used. 

Hunter's Screw consists of two screws, working one 
within the other, in such a way that as the larger descends 
the smaller ascends, though not quite so far. The differ- 
ence between the respective distances of the threads in the 
two screws determines how far on the whole the screw ad- 
vances. With Hunter's Screw, therefore, the power pro- 
duces a pressure as many times greater than itself, as the 
Fi §- 124 - difference between the respective 

distances of the threads in the two 
screws is contained in the circle 
described by the power. 

A is the larger screw, B W the smaller 

one. C D is the lever by which it is worked, 

and E F the stationary nut. The pressure 

is produced at W. If the threads of the 

larger screw are 1 inch apart, and those of 

the smaller 3 / 4 of an inch, the difference is 

J / 4 of an inch. Then, if the extremities of the 

lever describe a circle of 100 inches, the ad- 

W vantage will be equal to 100 divided by l / 4l 

hunter's screw. or 400 ; that is, a power of 1 pound applied 

at either end of the lever will produce a pressure of 400 pounds at W. 




the advantage gained by this machine to be calculated ? 264. For what purposes are 
6crews used ? 265. When great pressure is required, what difficulty attends the use 
of the screw? To remedy this, what ingenious contrivance is used? Describe Hun- 
ter's Screw. With this screw, how great a pressure does a given power produce ? 



THE ENDLESS SCREW. 



117 



By making the threads of the two screws nearly the same distance apart, 
an immense power is obtained without diminishing the size and strength of 
the threads. The action of the screw is of course proportionally slow, time 
being always lost as power is gained. 

266. The Endless Screw. — Instead of working in a 
nut, a screw is sometimes made to act on teeth cut in the 



Fig. 125. 



circumference of a wheel. In this 
case, the only motiDn of the screw 
is round its axis. The winch being 
turned, the threads of the screw 
catch the teeth of the wheel and 
move it forward. As fast as one 
tooth passes out of reach, another 
is caught ; and, the motion being 
thus continuous, the machine is 
called the Endless Screw. Its op- 
eration will be understood from 
Fig. 125, where it is combined 
with a wheel and axle for the purpose of lifting a weight. 




THE ENDLESS SCREW. 



EXAMPLES FOR PRACTICE. 

1. {See §204.) A lever of the first kind is 20 inches in length : the long arm 
is 15 inches ; the short arm, 5. How great a power will balance a weight 
of 112 pounds ? With the same lever, how great a weight will a power 
of 50 pounds balance ? 

2; A farmer, in forcing a stump from the ground, uses a crow-bar 6 feet long, 
which he rests on a stone five feet from the end where his hand is ap- 
plied. The resistance of the stump is equal to a weight of 500 pounds ; 
how great a pressure must he exert, to move it? 

3. A man weighing ISO pounds, and a boy of 60 pounds, are teetering on a 
board 12 feet long. That they may balance each other, how near must 
the man sit to the horse on which the board rests ? 

L A man whose strength enables him to use a pressure of 120 pounds, wishes 
to move a rock weighing 600 pounds with a lever of the first kind. What, 
must be the comparative length of the arms of the lever? 

If with his unaided strength he could move 120 pounds thirty feet in 
one minute, how long will it take him to move the rock with the lever 
the same distance ? 

Illustrate this with Fig. 124. How may an immense power be gained with Hunter's 
Screw ? 266. Describe the Endless Screw and its mode of operation. With what is 
it combined for lifting weights ? 



118 MECHANICS. 

5. (See § 207.) The short arm of a steelyard is 2 inches long ; at its end a 10- 

pound weight is suspended. How great a weight must be attached to 
the other end to balance it, the length of the steelyard being one foot ? 

6. (See § 213.) There is a compound lever formed of two simple ones, the first 

arms of which are 10 inches each, and the short arms 2 inches each. How 
great a weight at the extremity of the last short arm will be supported 
by a power of 1 pound at the other end? 
* (See § 215.) A lever of the second kind is 20 inches long; the weight is 5 
inches from the fulcrum. How great a power must be applied, to balance 
a weight of 112 pounds? 

8. With the same lever as in the last sum, how great a weight will a power 

of 50 pounds balance ? 

9. A is rowing with an oar 9 feet long, and has his row-lock 2 feet from his 

hand ; B rows with an eight-foot oar, and his row-lock is 1 foot from his 
hand. If they strike the water with an equal length of oar, which ex- 
erts the greater power on the boat ? 

10. (See § 217.) A man and a boy, at opposite ends of a bar 5 feet long, are 
carrying a 150-pound weight suspended between them. The boy can 
carry but 30 pounds ; how far from his end must the weight hang, to 
give him that portion of it, and the man the rest? 

11. Three men are bearing a weight suspended from a bar in the manner shown 
in Fig. 100. The single man at one end is twice as strong as each of the 
two at the other end. How must the weight be placed (the bar being 4 
feet long), that each may bear a part proportioned to his strength? 

i2. (See §219.) A lever of the third kind is 20 inches long; the power is 5 
inches from the fulcrum. How great must it be, to balance a weight of 
112 pounds? 

13. A pair of pincers is 6 inches long. How great a force must be applied, 
2 inches from the top, to overcome a resistance of 3 ounces? 

14. The distance of a man's hand from his elbow is 16 inches. The biceps 
muscle is inserted in his fore-arm 2 inches from the elbow. With how 
great power must the muscle act to sustain a weight of 56 pounds in the 
extended hand ? 

15. (See % 225.) The circumference of a wheel is 8 feet; that of its axle, 16 
inches. The weight, including friction, is 60 pounds ; how great a pow- 
er will be required to raise it ? 

16. The pilot-wheel of a boat is 3 feet in diameter ; the axle is 4 inches. The 
resistance of the rudder is 180 pounds, to which one-tenth of itself must 
be added for friction, &c. How great a power must be applied to the 
wheel, to move the rudder ? 

jl7. An axle one foot in circumference, fitted with a winch that describes a 
circle of 6 feet, is used for drawing water from a well. How great a power 
will it take to move 60 pounds of water, allowing one-tenth for friction ? 

18. Four men are drawing in an anchor that weighs 1,000 pounds, with a 
capstan. The barrel of the capstan has a radius of 6 inches. The circle 
described by the handspikes has a radius of 5 feet. How great a pres- 
sure must each of the four men exert, to move the anchor? 



EXAMPLES FOR PRACTICE. 119 

19. (See § 232.) With a fixed pulley, how great a power will it take to hoist 
a weight of 50 pounds, 20 percent., or one-fifth, being added for friction? 

20. (See § 238.) With a movable pulley, how great a power will it take to 
hoist a weight of 50 pounds, twenty per cent, being allowed for friction ? 

21. (See § 239.) With a fixed and a movable pulley, how great a power will 
it take to hoist a weight of 50 pounds, 4( per cent., or two-fifths, being 
allowed for friction ? 

2 '. ' See § 241.) With two fixed and two movable pulleys, how great a power 
will it take to hoist a weight of 50 pounds, 60 per cent., or three-fifths, 
being allowed for friction ? 

23. (See § 242.) How great a power will it take to hoist a weight of 100 pounds 
with one of White's Pulleys having five grooves in each block, 35 per 
cent., or seven-twentieths, being allowed for friction ? 

24. (See § 243.) With a system of six movable pulleys, having each its own 
rope, and arranged as shown in Fig. 115, how great a weight (including 
friction) will a power of 20 pounds raise ? 

25. With a similar system of five movable pulleys, how great a power will it 
take to balance a weight of 64 pounds, to which the friction of the pul- 
leys adds 50 per cent., or one-half of itself? — Am. 3 pounds. 

[64 + 32 = 96 2^ = 32 96 -r- 32 = 3 Answer.] 

26. (See % 247.) How great a power will be required to balance a weight of 
40 pounds (friction included), on an inclined plane, whose length is 8 
times its height ? 

27. (See § 253.) A weight of 1,500 pounds is to be raised with a wedge 
having a base of 60 inches and a height of 12 inches. How great must 
the power be ? — Ans. 300 pounds. 

28. I desire to raise a weight of 900 pounds with two similar wedges, 
as shown in Fig. 122. Each wedge has a base of 3 feet, and is 9 inches 
through at the head. How great a power must be applied to each ? 

29. A weight of 1,020 pounds is to be lifted iy 2 feet. The greatest power 
that can be applied is 255 pounds. Give the dimensions of the wedge. 

30. (See % 257.) Of two wedges 4 inches thick at the head and respectively 6 
and 8 inches long, which can be driven into a log the more easily? 
Which will break the sooner, both being made of the same material ? 

31. (See % 262.) How great a pressure (including friction) will be exerted by 
a power of 15 pounds applied to a screw whose head is 1 inch in circum- 
ference, and whose threads are one-eighth of an inch apart ? 

32. A book-binder has a press, with a screw whose threads are one-third of an 
inch apart, and a nut worked by a lever which describes a circle of 8 feet. 
How great a pressure will a power of 5 pounds applied at the end of the 
lever produce, the loss by friction being equivalent to 240 pounds ? 

^3. (See §265.) How great a pressure is produced by a power of 1 pound 
with one of Hunter's Screws, worked by a lever which describes a circle 
of 75 inches ; the threads of the larger screw being half an inch apart 
and those of the smaller one-third of an inch, 33 1 / 3 per cent., or one-third, 
of the pressure being deducted for friction ? 



120 MECHANICS. 



CHAPTER IX. 

MECHANICS (CONTINUED). 

WHEELWORK. CLOCK AND WATCHWORK. 

267. All machines, however complicated, are combina- 
tions of the six simple mechanical powers described in the 
last chapter. The chief objects in combining them are to 
gain a sufficient degree of power, and to give such a direc- 
tion to the motion as will make the machinery do the work 
required. 

Wheel work. 

268. The wheel enters more largely into machinery than 
any other of the Mechanical Powers. 

269. Several wheels combined in one machine are called 
a Train. 

270. In a train of two wheels, the one that imparts the 
motion is called the Driver ; the one that receives it, the 
Follower. 

271. Modes of Connection. — There are three ways in 
which motion may be transmitted from one wheel to an- 
other: — 1. By the friction of their circumferences. 2. By 
a band. 3. By teeth on their outer rim. 

272. Friction of the Circumferences. — One wheel may 
move another by rubbing on its circumference, or outer 
rim. The wheels are so placed that their rims touch, and 
one of them is set in motion. The circumference of each 



267. Of what are all machines combinations? What are the chief objects in com- 
bining them? 26S. Which of the mechanical powers enters most largely into ma- 
chinery? 269. What is meant by a Train of wheels? 270. In a train of two wheels, 
which is the Driver? Which, the Follower? 271. In how many ways may motion 
be transmitted from one wheel to another? Mention them. 272. How may one 
wheel be made to move another by rubbing on its circumference ? What is the ad- 



WHEELWOEK. 



121 



having been previously roughened, friction prevents the 
moving wheel from slipping over the one at rest, and mo- 
tion is imparted to the latter. Wheels thus connected 
work regularly and with little noise, but will not answer 
when a great resistance is to be overcome, and hence arc 
not much used. 

273. Bands. — One wheel may be made to move an 
3ther by means of a band passed round both circumfer 
ences. Such a band is known as a Wrapping Connector. 
It is also called an Endless Band, because, its ends being 
joined, we never seem to reach them, though the motion is 
continuous in the same direction. The band must be 
stretched so tight that its friction on the wheels may be 
greater than the resistance to be overcome. 

Fig. 126 shows how wheels are connected by an Fig. 126. 

endless band. If the follower is to turn in the same 
direction as the driver, the band is passed over it 
without crossing, as in A ; if in the opposite direction, 
the band is crossed, as in B. 

274. The bands, or belting, used for this purpose 
are generally made of leather or prepared india rub- 
ber. The wheels may be far apart, if necessary ; and 
on this account, as well as because a great amount of 
power may thus be transmitted, the wrapping con- 
nector is much used. The motion imparted is exceed- 
ingly regular, any little inequalities being corrected 
by the stretching of the band. Fig. 127. 

275. Fig. 127 shows the different forms given to 
the circumferences of wheels, in order that the band 
may not slip off. A's circumference is concave, or 
hollows towards the centre, with a rim on each side. 
B's is the same, with a row of pins down the centre. 
C's circumference is even across, with a rim on each 
side. D has no rim, but bulges out in the centre, so c 
that when the band tends to approach one side it is B 
pulled back by the tightening on the other. 




J^J 



vantage, and what the disadvantage, of this mode of connection ? 273. What is a 
Wrapping Connector ? What other name is given to it, and why ? How tight must 
the band be ? In passing from the driver to the follower, when is the band crossed, 
and when not ? 274. Of what are endless bands usually made ? By what advantages 
is their use attended ? What renders the motion imparted by wrapping connectors 
exceedingly regular ? 275. Describe the different forms given to the circumferences 
of wheels on which a wrapping connector is to act. 276. What is the third way in 

6 



122 



MECHANICS. 




Fig. 128. 276. Teeth. — One wheel may be made to 

move another by means of teeth on the circum- 
ference of each. A toothed wheel is shown in 
Fig. 128. 

277. Small toothed wheels combined with 
large ones are called Pinions, and their teeth Leaves. 

278. Two or more wheels connected by teeth are called 
Gearing. When so arranged that the teeth work in each 
other, they are said to be in gear; and when not, out of 
gear. 

Fig. 129. Figure 129 shows a 

train of wheels and pin- 
ions in gear. To find 
how great a weight will 
be balanced by a given 
power with such a 
train, multiply the pow- 
er successively by the 
number of teeth on the 
wheels, and divide by 
the product of the num- 
ber of teeth on the pin- 
tkain of wheels and pinions. ions. For instance, in 

Fig. 129, let the first large wheel have IS teeth, the second 18, the third 27, 
and the fourth 27 ; and let each pinion have 9 teeth. Then (leaving friction 
out of account) a power of 2 pounds will balance a weight of 72 pounds. For 

2 X 18 X 18 X 27 X 27 = 472392 

9X9X9X9 = 6561 

472392 divided by 6561 = 72 

279. Kinds of Toothed Wheels. — There are three 
kinds of toothed wheels ; viz., Spur-wheels, Crown-wheels, 
and Bevel-wheels. 

280. Spur-wheels. — Spur-wheels have their teeth per- 
pendicular to their axes, as shown in Fig. 129. 

The teeth are either made in one piece with the rim, or 




which one wheel may be made to move another ? 277. What are Pinions ? What 
are the teeth of pinions called ? 278. What is Gearing? When are wheels said to be 
in gear t When are they said to be out of gear ? What does Fig. 129 represent ? 
With such a train, how do you find how great a weight will be balanced by a given 
power? Give an example. 279. How many kinds of toothed wheels are there? 
Name them. 280. Describe Spur-wheels. How are the teeth made ? What are 



WHEELWORK. 



123 



consist of separate pieces set into the rim. In the latter 
case, they are called Cogs. 

In mills, Cog-wheels are gen- 
erally used with Trundles, or Lan- 
terns, as represented in Fig. 130. 

A is a large cog-wheel. B is a trundle, 
.consisting of two parallel discs and an inter- 
vening space traversed by round pins called 
Staves, so arranged as to receive the cogs of' 
the other wheel. 

Mill-wheels are generally made of cast- 
iron ; but they are found to work most smooth- 
ly when one of them has wooden instead of 
iron teeth. Wooden teeth are therefore often 
set in the larger one, which is then called a 
Mortice-wheel. 

281. Crown-wheels. 
parallel to their axes. 

Fig. 131. 




COG-WHEEL AND TRUNDLE. 



Crown-wheels have their teeth 




Fig. 132. 




CBOWN-WHEEL AND PINION. 



HAND-MILL. 



Fig. 131 represents the contrate-whe^l and pinion of a watch. B, whose 
teeth run the same way as its axis, is a crown-wbeel. A, whose teeth are at 
right angles to its axis, is a spur-wheel. 

Fig. 132 shows how a crown-wheel worked by a winch is combined with 
a trundle in a hand-mill used in Germany and Northern Europe. The crown- 
wheel moves vertically, but it communicates a horizontal motion to the trun- 
dle, which in turn imparts it to the mill-stone. 

282. Bevel-wheels. — Bevel-wheels are wheels whose teeth 



Cogs? In mills, with what are cog-wheels generally used? Describe a Trundle 
Of what are mill-wheels generally made ? What is said of their Teeth ? What is a 
Mortice- wheel ? 281. Describe Crown-wheels. What does Fig. 131 represent ? De- 
scribe the hand-mill represented in Fig. 132. 282. What are Bevel-wheels' What 



124 



MECHANICS. 



form any other angle with 
their axes than a right angle. 

A pair of bevel-wheels 
in gear are shown in Fig. 
133. 

283. Rack and Pin- 
ion. — Circular motion is 
converted into rectilinear 
(that is, motion in a straight 
line) by means of the rack bevel-wheels. 
and pinion, represented in Fig. 134. As 
the pinion A revolves, its teeth work in 
Fig. 134. those of the rack 

B C, moving it for- 
ward in a straight line. 



Fig. 183. 




284. Forge-Hammer. — A toothed 



Fig. 135. 



BACK AJS1> P1MON. 

wheel may produce an alternate up-and- 
down motion, as in the case of the Forge- 
hammer, represented in Fig. 135. 

The wheel is so placed that its teeth successively 
come in contact with the handle of the hammer, which 
turns on a pivot. As the wheel revolves, a long 
tooth carries the lower end of the handle down and 
raises its head. As soon as the tooth releases the handle, the head of the 
hammer falls on the anvil by its own weight. A new tooth then comes into 
play, and the operation is repeated. 

285. Cranks. — The Crank is much used in machinery 
for converting circular motion into rectilinear, or rectilinear 




THE FORGE-HAMMER. 



Fig. 136. 



into circular. It has different forms, but is 
generally made by bending the axle in the 
way represented in Fig. 136. As the wheel 
to which it is attached turns, the crank A 
also revolves, and causes the rod B, with 
which it is connected, to move alternately 

xa.iu '.ria.j\n.. 

up and down. 

does Fig. 133 represent ? 283. How may circular motion be converted into rectilin- 
ear? Describe the working of the Rack and Pinion. 284. What kind of motion does 
a toothed wheel produce in the case of the forge-hammer ? Explain the wo king of 
the forge-hammer. 285. For what is the Crank used ? Describe its usual form, and 




c > — ^ 



THE CRANK. 125 

The point at which the rod stands at right angles to the 
axle (as in the Figure) is called the Dead-point. Two dead- 
points occur in each revolution. When at either, the crank 
loses its power for the instant ; but the impetus carries it 
along, and as soon as the dead-point is passed it again be- 
gins to act. 

286. Another form of the crank is exhib- Fig. 137. 

ited in Fig. 137, which shows how a wheel is 
moved by a treadle-board worked by the foot. 
A is the treadle ; B C is a cord passed round 
the pulley D, and attached to the crank E, 
which is connected with the axle of the wheel 
F. When the foot bears the treadle-board 
down, the end of the crank is raised to its 
highest point. Here it would remain if the 

„, . rt. v. J u ' x. ±1. * a CRANK AND TREADLE. 

foot were kept on the board ; but, the foot 

being removed, the impetus of the wheel carries the crank round again to its 
lowest point, raising in turn the end of the treadle-board. The foot is now 
applied again with the same effect as before, and continuous motion is thus 
imparted to the wheel. 

287. Fly-wheels. — The motion of machinery must be 
even and regular. Both power and resistance must there- 
fore act uniformly; if either increases too rapidly, the sud- 
den strain is apt to break some part of the works. To 
prevent this, the fly-wheel is used. 

The fly-wheel appears in various forms, but generally 
consists of a heavy iron hoop with bars meeting in the cen- 
tre. It is set in motion by the machinery, and by reason 
of its weight acquires so great a momentum that irregu- 
larities either in power or resistance, unless long continued, 
have but little effect. If, for instance, the power ceases to 
act for a moment, or the resistance suddenly increases or 
diminishes, the great momentum of the fly prevents the 
motion of the machinery from varying to any great extent. 
288. The fly-wheel also accumulates power, and thus enables a machine 
to overcome a greater resistance than it could otherwise do. The power, 

explain its operation. What is meant by the Dead-point of the crank ? What is said 
of the crank at its dead-point ? 286. What does Fig. 137 represent ? Explain the op- 
eration of the crank and treadle. 287. For what is the Fly-wheel used? Of what 
does it generally consist ? Explain how the fly-wheel prevents irregularities of mo- 
tion. 288. For what other purpose is the fly-wheel used ? How does the fly-wheel 



126 MECHANICS. 

allowed to act on the fly alone for a short time, gives it an immense momen- 
tum ; and this momentum directly aids the power, when the machine is ap- 
plied to the required work. 

Clock and Watch Work. 

289. One of the commonest and most ingenious appli- 
cations of wheel work is exhibited in clocks and watches. 

290. History. — The advantages of combining wheels 
and pinions were partially known as far back as the time 
of Archimedes ; yet they were comparatively little used in 
machinery, and not at all for the measurement of time. 

Instead of clocks and watches, consisting of trains of wheels, the ancients 
used the sun-dial, and clep'-sy-dra or water-clock. The former indicated the 
hour by the position of the shadow cast by a style, or pin, on a metallic plate ; 
the latter, by the flow of water from a vessel with a small hole in the bottom. 
The dial was of course useless at night ; and neither it nor the clepsydra, 
however carefully regulated, could measure time with any great degree of 
accuracy. 

Even Alfred the Great, 985 years after Christ, had no suitable instrument 
for measuring time. To tell the passing hours, he used wax candles twelve 
inches long and of uniform thickness, six of which lasted about a day. Marks 
on the surface at equal intervals denoted hours and their subdivisions, each 
inch of candle that burned showing that about twenty minutes had passed. 
To prevent currents of air from making his candles burn irregularly, he en- 
closed them in cases of thin, transparent horn. But, after all, King Al- 
fred's candles were only approximate measurers of time. 

291. Clocks moved by weights were known to the Sar- 
acens as early as the eleventh century. The first made in 
England (about 1288 a. d.) was considered so great a work 
that a high dignitary was appointed to take care of it, and 
paid for so doing from the public treasury. The usefulness 
of clocks was greatly increased by the application of the 
pendulum, which was made about the middle of the seven- 
teenth century. 

Watches seem to have been first made in the six. 

aid the power? 289. In what do we find one of the most ingenious applications of 
wheel- work ? 290. What is said of the. knowledge of wheel-work possessed by the 
ancients? What did the ancients use for the measurement of time? How did the 
sun-dial indicate the hour ? How, the clepsydra ? What is said of the accuracy of 
these instruments ? How did Alfred the Great measure time ? How did he keep off 
currents of air? 291. When were clocks moved by weights first made by the Sara- 
cens? When was the first made in England? How was this clock regarded ? What 



CLOCK AND WATCH WORK. 127 

teenth century, though it is not known who was their in- 
ventor. For a time they were quite imperfect, requiring to 
be wound twice a day, and having neither second nor min- 
ute hand. The addition of the hair-spring to the balance, 
by Dr. Hooke, in 1658, was the first great improvement. 
Others have since been devised ; and chronometers (as the 
best watches, manufactured for astronomers and naviga 
tors, are called) are now made so perfect as not to deviate 
a minute in six months, even when exposed to great varia- 
tions of temperature. 

292. Clock-work. — In clocks, except such as are moved 
by springs similarly to watches, the moving power is a 
weight ; to which, when wound up, gravity gives a constant 
downward tendency. In its effort to descend, it sets in 
motion a train of wheels and pinions ; and they move the 
hands which indicate the hours and minutes on the face. 

The motion of the wheels, though caused by 
the weight, is regulated by the pendulum and an 
apparatus called the Escapement, shown in Fig. 
138. The anchor-piece ABC moves with the pen- 
dulum. As the latter vibrates, the pallets B, C, 
are alternately raised far enough to let one tooth 
of the scape-wheel pass, its motion at other times 
being checked by the entrance of one of the 
pallets between the teeth. Hence, though the 
weight is wound up, the clock does not go till 
the pendulum is set in motion. If the pendu- 
lum and escapement are removed, the weight 
runs down unchecked, turning the various wheels THE escapement. 

with great rapidity. The motion of the wheels is thus made uniform by the 
pendulum ; and by shortening or lengthening it we can make the clock go 
faster or slower. 

293. Watch-work. — In a watch, there is no room for 
a weight or pendulum ; hence a spring, called the main- 

grreatly increased the usefulness of clocks ? When were watches first made ? What 
was the character of those first constructed ? What was the first great improve- 
ment ? What is said of the chronometers made at the present day ? 292. What is the 
moving power in clocks ? How does the weight set the clock in motion ? How is 
the motion of the wheels regulated ? Explain, with Fig. 138, how the Escapement 
regulates the motion. If the pendulum and escapement are removed, what is the 
oonsequep,ce ? How is the clock made to go faster or slower ? 293. In a watch, what 




128 MECHANICS. 

spring, is substituted for the former as a moving power, 
while the balance and hairspring take the place of the lat- 
ter as a regulator. 

The main-spring is either fixed to an axle capable of revolving, as shown 

at P in Fig. 140, or is contained within a hollow barrel, connected by a chain 

with a conical axle, called the fusee, represented in Fig. 139. A is the barrel, 

Fig. 139. within which and out of sight is the 

II I main-spring, having one end attached to 

"""" the inner surface of the barrel, and the 




other fastened to a fixed axle passing 
through the barrel. B is the fusee. 
The watch is wound up with a key, 

THE r L Sth, •*■ ** 

applied to the square projecting from 
the fusee. By turning the square the chain is drawn off" from the barrel and 
wound round the fusee. The barrel is thus turned till the spring in the in- 
side is tightly coiled. This spring, by reason of its elasticity, tends to un- 
coil, and in so doing moves the barrel round, drawing off the chain from the 
fusee, and winding it again around the barrel. The fusee is thus turned, and 
carries with it the first wheel of the train, which imparts motion to all the 
rest. When the spring has uncoiled itself, the chain, being entirely wound 
round the barrel, ceases to move the fusee, and all the wheels come to rest. 
The watch is then said to run down. 

The reason of the peculiar shape of the fusee is this. The power of the 
spring is proportioned to the tightness with which it is coiled, and hence is 
greatest when the watch is first wound. The chain is consequently then 
made to act on the smallest part of the fusee ; because, the nearer to the axis 
the force is applied, the less its power of producing motion. As the spring 
gradually uncoils, its power is weakened and it is made to act on a larger 
part of the fusee. By thus adjusting the size of the fusee to the varying 
power of the spring, a uniform effect is secured. 

294. An escapement similar to that used in clocks connects the moving 
power with the balance. To the latter, also, a very fine spiral spring is at- 
tached, which is fastened at its other end to a fixed support. The watch is 
regulated by shortening or lengthening this spring, the balance being made 
to vibrate faster or slower accordingly. 

295. The works of an ordinary watch are shown in Fig. 
140. For convenience of inspection, they are arranged in 
a line, and the distance between the two plates, and also 
between the upper plate and the face, is increased. 

takes the place of the weight, and what of the pendulum ? What two ways are there 
of fixing the main-spring ? Explain Fig. 139. How is the watch wound up ? Ex- 
plain the working of the fusee. When does the watch run down, and why does motion 
then cease ? What is the reason of the peculiar shape of the fusee ? 294. What con- 
nects the moving power with the balance ? What is attached to the balance ? How 



WATCH-WORK. 



129 



Fig. 140. 




WORKS OF A MATCH. 



OP is the main-spring, attached 
to its axle, without a fusee. The un- 
coiling of the spring carries the axle 
round, and with it the great wheel N. 
N works in the pinion a, and by turn- 
ing it turns also the centre-wheel M on 
the same axis, so called from being in 
the centre of the 
*5x watch. M turns 
the pinion b and 
the third wheel 
L, which in turn 
works in the pin- 
ion c and causes 
the second or con- 
trate-wheel R, on 
the same axis, 

to revolve. R works in the pinion d and carries round the balance or crown 
wheel C, which is on the same axis with it. 

The saw -like teeth of the balance-wheel are checked (as in the escape- 
ment of a clock) by the pallets p,p, which are projecting blades on the 
verge of the balance A. The hair-spring, fastened at one end to a fixed sup- 
port, and at the other to the balance, may be shortened by the curb or reg- 
ulator, if the watch goes too slow, or lengthened if it goes too fast, thus con- 
trolling the motion of the balance and consequently that of the other wheels. 
296. The force of the main-spring is so adjusted as to make the great 
wheel N revolve once in four hours. The spring generally turns it seven or 
eight times round before it is uncoiled, so that with one winding the watch 
runs twenty-eight or thirty-two hours. The great wheel X has forty-eight 
teeth, the pinion a but twelve ; so that a and the centre-wheel M revolve once 
every hour, and their axle, carried through to the face, bears the minute- 
hand. 

Between the face and the upper plate is a train of pinions and wheels con- 
nected with the axle of the centre-wheel. They are so adjusted that the wheel 
V revolves once in twelve hours. V carries the hour-hand. It is attached 
to a hollow axle, through which the axle of the centre-wheel passes to carry 
the minute-hand. 

297. Thus we see that the works of a watch are nothing 
more than an ingenious combination of wheels, moved by a 
spring and regulated by a balance. The arrangement of the 



is the watch regulated? 295. What does Fig. 140 represent? With the aid of Fig. 
140, describe the works of a watch and their mode of operation. How is the watch 
regulated? 296. How great a force ia generally given to the main-spring? How 
long does the watch run with one winding ? Explain the arrangement of the minute- 
hand. Explain that of the hour-hand. 297. Of what, as we have seen, do the work* 

9 



/ 130 HYDROSTATICS. 

wheels and pinions is such, that there is a constant increase 
of velocity and a corresponding loss of power. The great 
wheel, which begins the train, revolves once in four hours ; 
the balance, which closes it, revolves in one-fifth of a sec- 
ond ; but the force of the spring becomes so attenuated 
by the time it reaches the balance, that the slightest addi- 
tional resistance there, a particle of dust or even a thicken- 
ing of the oil used to prevent friction, deranges, and may 
stop, the action of the whole. 



CHAPTER X. 
MECHANICS (CONTINUED). 

HYDROSTATICS. 

298. Hydrostatics and Hydraulics are branches of Me- 
chanics that treat of liquids. 

Hydrostatics is the science that treats of liquids at rest. 
Hydraulics is the science that treats of liquids in mo- 
tion, and the machines in which they are applied. 

299. The principles of Hydrostatics and Hydraulics are 
equally true of all liquids ; but it is in water, which is the 
commonest liquid, that we most frequently see them ex- 
hibited. 

Water abounds on the earth's surface. It covers more than two-thirds of 
the globe, and constitutes three-fourths of the substance of plants and ani- 
mals. 

300. Nature of Liquids. — Liquids differ from solids in 
having but little cohesion. 



of a watch consist? What is said of the arrangement of the wheels and pinions? 
What is the comparative velocity of the great wheel and the balance ? What is saicl 
■>f the force of the spring by the time it reaches the balance ? 

298. What sciences treat of liquids ? What is Hydrostatics ? What is Hydraulics ? 
299. What is said of the principles of hydrostatics and hydraulics ? How much of 
the globe is covered with water ? How much of the substance of plants and animals 
consists of water ? 300. In what respect do liquids differ from solids ? What shows 



NATURE OF LIQUIDS. 131 

Cohesion is not entirely wanting in liquids, as is proved by their parti- 
cles' forming in drops ; but it is so weak as to be easily overcome. Thick 
and sticky liquids, like oil and molasses, have a greater degree of cohesion 
than thin ones, like water and alcohol. 

301. Liquids were long thought to be incompressible, 
but experiment has proved the reverse. Submitted to a 
pressure of 15,000 pounds to the square inch, a liquid loses 
one-tvventy-fourth of its bulk. Were the ocean at any point 
a hundred miles deep, the pressure of the water above on 
that at the bottom would reduce it to less than half its 
proper volume. 

302. To distinguish them from the gases, liquids are 
often called non-elastic fluids ; yet they are not devoid of 
elasticity. 

To prove this, after compressing a body of water, remove the pressure, 
and it will resume its former bulk. Again, if a knife-blade be brought in 
contact with a drop of water hanging from a surface, the drop may be elon- 
gated by slowly drawing away the blade ; but it immediately returns to its 
original shape, if the blade is entirely removed without detaching the drop 
from the surface. 

Law of Hydrostatics. 

303. Water at rest always finds its level. 

No matter what the size or shape of a body of water may be, its surface 
has the same level throughout ; that is, it is equally distant at every point from 
the earth's centre. Accordingly, the surface of the ocean is spherical ; and 
this we know to be the case from always seeing the mast of a vessel approach- 
ing in the distance before we see the hull. In small masses of liquids, no 
convexity is perceptible ; and we may consider their surfaces as perfectly flat. 

304. The tea-pot affords us a familiar illustration of this law. The tea 
always rises as high in the spout as in the body of the pot ; and, if the body 
is higher than the spout, it will pour out from the latter when the pot is 
filled. 

So, let there be a number of vessels having communication at their bases, 
as shown in Fig. 141. If water be poured into any of them, it will rise to 

that cohesion is not entirely wanting in liquids ? What liquids have the most cohe- 
sion ? 301. What is said respecting the compressibility of liquids ? If the ocean were 
& hundred miles deep, what would be the consequence of the pressure ? 302. What 
are liquids often called, to distinguish them from gases? Is the name strictly correct? 
Prove that liquids are elastic ? 303. What is the great law of Hydrostatics ? What 
do we mean, when we say that a body of water has the same level throughout ? What 
sort of a surface must the ocean have ? What evidence is there of this ? How may 
w-e regard the surfaces of small bodies of liquids ? 304. Show how the tea-pot illus- 



132 



HYDROSTATICS. 



Fig. 141. the same level in all, 

no matter how they 
may differ in shape or 
size. In like manner, 
if there be subterra- 
neous connection be- 
tween a river affected 
by the tide and pools 
near its banks, the wa- 
ter in the pools will 
rise and fall simulta- 
neously with that in the river. 

305. We take advantage of this law in supplying cities 
with water from elevated ponds or streams. The water 
may be conveyed in pipes any distance, may be carried be- 
neath deep ravines or the beds of rivers, and when released 
from the pipe at any point will rise to the level from which 
it started. 

Fig. 142. 





Thus, in Fig. 142, the pond A is made to supply the house D with water 
by means of pipes carried down into the valley, under the stream B and over 
the bridge C. In the house it will reach the level of the pond from which it 
was taken, shown by the dotted line. 

Fountains formed by tapping the pipe ac any point, rise, theoretically, to 
the same level, as seen in the plate, but are prevented from quite reaching 
it by the resistance of the air and the check which the ascending stream re- 
ceives from the falling drops. 

306. The ancient Romans appear to nave Known that water conducted 
in pipes will find its level ; yet so difficult did they find it to make water- 



trates this law. Illustrate it with Fig. 141, How does this law apply in the case of 
pools connected with tide-water ? 305. To what practical purpose is this principle 
applied ? Illustrate this with Fig. 142. How high will fountains formed by tapping 
the pipe rise? 306. How did the ancient Eomans convey their supplies of water? 



ARTESIAN WELLS. 133 

tigty joints, that, instead of employing pipes, they conveyed their water 
through vast level aqueducts, bridging at an immense expense such ravines 
and valleys as lay in their course. In modern times, iron pipes laid beneath 
the surface, however much it may be depressed, accomplish the same object 
with much less cost, the water always rising to its original level when al- 
lowed to do so. The lower the pipes are sunk, the stronger they should be ; 
for the upward pressure of the water, tending to resume its level, increases 
fc proportion to the depth. 

307. Artesian Wells. — It is on this principle, also, that 
Artesian Wells are made. They are so called from the 
province of Artois [ahr-twah'~\, in France, the first district 
of Europe where they were extensively introduced, though 
known to the Chinese for centuries. 

The outer crust of the earth consists of different strata, or layers ; some 
of which (rock and clay, for instance) are impervious to water, and others 
not (such as gravel and chalk). If a stratum which allows water to flow 
through it is enclosed, after leaving the surface, between two impervious 
layers, and thus descends to a lower level, the water received by this stratum 
at the surface, unable to pass out above or below, collects in it throughout its 
whole length. Let an opening then be made at any point into this reservoir 
through the impervious stratum above, and the water will at once rise to 
find its level. 

Such openings are Artesian wells. They have been carried in some cases 
a third of a mile below the surface ; and so abundant is their supply of water 
that a single well of this kind near Paris has been computed to yield more 
than 700,000 gallons daily. The elevated end may be several hundred 
miles distant ; it matters not how far. Parts of Algeria bordering on 
Sahara, once considered an irreclaimable desert, have been supplied with 
water, and thus rendered habitable, by means of Artesian wells. 

308. Springs. — Springs have a similar origin. The rain 
drunk up by the earth's surface gradually sinks, till it 
reaches an impervious stratum. Along this it runs, re- 
ceiving additions as it goes, till it finds vent in some nat- 
ural opening. 

In ordinary wells, the water does not rise to the earth's surface, because 
it does not come from an elevated stratum. 



Why did they not employ pipes ? What precaution must he taken, in consequence 
of the upward pressure of the water? 307. What wells are made on this principle? 
Why are Artesian Wells so called? Explain their working. How low have they 
been carried ? How much water does the well near Paris supply ? How far off may 
the elevated end of the stratum he? What has been done in parts of Algeria? 
808. Explain the origin of springs. Why does not the water rise in ordinary 



134 



HYDROSTATICS. 



309. Locks. — We are enabled to run canals through un- 
even tracts by taking advantage of the fact that water al- 
ways finds its level. If the bottom of the canal were not of 
a uniform grade, the water would run towards the lower end 
and inundate the surrounding country. When, therefore, 
the ground is uneven, the canal is built in sections, each level 
in itself, but of a different grade from the one next to it, 
with which it is connected by a compartment called a Lock. 

Fig. 148. 
D 




Let AB represent a canai, the upper section of which, A, is fifteen feet 
higher than the lower section B. A boat is passed from one to the other by 
means of the lock C, which communicates with either section, as may be de- 
sired, by opening sliding valves in the lock-gates D, E. When a boat is going 
down, the gate E is closed and D is opened till the water in the lock assumes 
the same level as in A. The boat is then brought into the lock ; the gate D is 
closed and E is opened. The water, gradually sinking in the lock, bears the 
boat along with it till it reaches the same level as in B. In going up, the op- 
eration is reversed. The boat having passed from B into the lock, E is closed 
and D opened. The water rushes in to find its level, and the boat is raised 
till it stands at the same height as the water in A. 

310. The Spirit Level. — The Spirit Level, an instru- 
ment much used by surveyors, masons, and others, operates 
Fig. 144. on this same principle. It consists 

of a glass tube (see Fig. 144), slight- 
ly curved, and nearly filled with 
the spirit-level. colored alcohol, just enough air be- 
ino* allowed to remain in it to form a bubble. The tube is 
then closed, and fixed in a wooden or metallic case. 

On being applied to a surface, if the latter is perfectly level, the air-bub- 
ble will rest midway of the tube, in its highest point, which has been found 



wells ? 309. How are we enabled to run canals through uneven tracts of coun- 
try ? With the aid of Fig. 143, show the workings of a Lock. 310. What is the 



PRESSURE OF LIQUIDS. 



135 



by previous experiment and marked. If the bubble rests in any other place, 
it shows that one end of the tube is higher than the other, and consequently 
that the surface on which it rests is not level. 

The tube is sometimes made of a different form, and nearly filled with wa- 
ter instead of alcohol ; the instrument is then known as the Water Level. 



Fig. 145. 



Pressure of Liquid*. 

311. First Law. — Liquids , subjected to pressure, trans* 
mit it undiminished in all directions. 

Solids transmit pressure only in the line in which it is 
exerted ; liquids transmit it in every direction. This is 
proved by experiment. 

In Fig. 145, A represents a glass vessel of water, to the 
neck of which a piston, B, is tightly fitted. Tubes are 
inserted at intervals through orifices in the sides. As the 
piston is driven down, the pressure is felt alike at all points 
of the vessel, as is shown by the flow of the water from 
the tubes. 




312. Second Law. — Liquids, influenced 
by gravity alone, press in all directions. 

Bore a hole in the bottom of a pail filled with water ; 
the water rushes out — this proves its downward pressure. 

Bore a hole in the side of the same pail : the water 
rushes out — this proves its lateral pressure. 

Bore a hole in the bottom of a boat ; the water rushes 
in — this shows its upward pressure. 

313. Third Law. — The pressure of liquids in every di- 
rection is proportioned to their depth. 

The downward pressure of liquids increases with their depth. To prove 
this, take four tubes of equal diameter, and over one end of each tie a piece 
of very thin india rubber. Fill them with water to different heights, say 5, 
10, 20, and 30 inches. The india rubber will be distended the most in the 
one containing the greatest depth of liquid, 

The lateral pressure of liquids increases with their depth. Hence dams 



Spirit Level? Of what does it consist? How is the spirit level used? What is tho 
Water Level? 311. What is the first law relating to the pressure of liquids? What is 
:lie difference between solids and liquids in this respect? Illustrate this law with 
Fig. 145. 312. What is the second law relating to the pressure of liquids? Prove the 
downward pressure of liquids. Prove their lateral pressure. Prove their upward pres- 
sure. 813. What is the third law relating to the pressure of liquids ? What experi- 
ment proves that the downward pressure of liquids is proportioned to their depth? 



136 



HYDROSTATICS. 



Fig. 146. 




and sea-walls should increase in strength towards their bases. On the same 
principle, barrels holding liquids should be more securely hooped at bottom 
than at top. 

The upward pressure of liquids increases with their 
depth. This is shown by the experiment represented in 
Fig. 146. A B is an open tube, ground perfectly smooth on 
the lower end. C is a plate of lead attached to a string. 
Pass the string through the tube, and with it keep the lead 
plate close against the ground end ; then introduce the 
whole into a deep vessel of water. When it has descended 
an inch or two, let go the string, and the lead will sink. Let 
it go near the bottom of the vessel, and, as shown in the 
Figure, the lead will be supported by the water. The up- 
ward pressure has therefore increased with the depth. 

314. At great depths the pressure of water becomes im- 
mense; neither divers nor fish can endure it. Strong glass 
bottles, empty and tightly corked, are often let down with cords at sea, and 
the pressure is generally sufficient to break them at a depth of 60 feet. If 
the bottle does not break, either the cork is driven in or water 
Fig. 147. enters through its pores. The hardest wood, sunk to a great 
depth, has its pores so thoroughly filled with water as to become 
incapable of rising. Hence, when a ship goes down at sea, her 
timbers are never seen again. 

315. This law leads to wonderful results. Ef- 
fects almost incredible may be produced by an in- 
significant body of liquid so disposed as to have 
considerable depth. 

We may, for example, burst a stout cask with a few ounces of 
water. Having filled the cask with water and inserted in its. top 
a long tube communicating with the inside, we may force the 
staves asunder, however tightly hooped, by simply pouring wa- 
ter into the tube. 

316. Similar effects are often produced in nature. Let D (see 
Fig. 148) be a mass of rock through which runs a long crevice, 
A B, communicating with C, a large cavity below, full of water, 
and having no outlet. When a shower fills the crevice, so great 
• pressure may be generated as to rend the rock in fragments. It is in this 
way that many of the great convulsions of nature are produced. 



What should be the strongest part of dams, sea-walls, and barrels, — and why ? De- 
scribe the experiment which proves that the upward pressure of liquids increases 
With their depth. 314. What is said of the pressure of water at great depths ? Whal 
experiment is often made with strong glass bottles ? What is the effect of this pres 
sure on wood sunk to a great depth ? 315. How may wonderful effects be produced 
by an insignificant body of liquid? How, for example, may a cask be burst? 
316. What similar effect is produced in nature ? 317. What is meant by the Hydro- 



PRESSUKE OF LIQUIDS. 



137 




317. Hydrostatic 
Paradox. — Pressure 
being proportioned 
to depth alone, a very 
small quantity of li- 
quid may balance any 
quantity, however 
great. This princi- 
ple is called the Hy- 
drostatic Paradox. 
Improbable as it appears at first, its truth is proved in va- 
rious ways. 

In Fig. 149, let A be a vessel holding 50 gallons, 
and B a tube of the same height, communicating with 
A, and having a capacity of one gallon. Water poured 
in either rises to the same height in both. When both 
are full, the pressure of the one gallon in the tube must 
be as great as that of the 50 gallons in the vessel ; oth- 
erwise, the latter would force its way into the tube and 
cause the water there to overflow. 

318. Rule for finding the Pressure on 
the Bottom of a Vessel. — To find the pres- 
sure of a body of liquid on the bottom of 
the vessel containing it, multiply its height into the area 
of the vessel's bottom. 




Fisr. 150. 



■BL—jBK J™ 



According to this rule, different 
quantities of liquid may produce equal 
pressure. In Fig. 150, let A, B, and C 
be three vessels having equal bases, 
and containing the same depth, though 
different quantities, of liquid ; then the 
pressure on their bottoms will be equal. 

319. Hydrostatic Bellows. — Interesting experiments 
may be performed with the Hydrostatic Bellows, repre- 
sented in Fig. 151. 



static Paradox ? Prove the truth of the paradox with the apparatus represented in 
Fig. 149. 318. What is. the rule for finding the pressure of a body of liquid on the 
bottom of the vessel containing it ? Explain how different quantities of liquid may- 
produce equal pressure. 319. Describe the Hydrostatic Bellows, and the experiment 



138 



HYDROSTATICS. 



Fig. 151. A metallic pipe, about four feet long, is screwed into a water- 

**# tight apartment, formed of two circular pieces of board fastened 
together with a broad leather band. As water is poured into the 
pipe, the top of the bellows rises, and with such force as to lift 
heavy weights placed upon it. When both pipe and bellows are 
full, the latter will support from three to four hundred pounds. 
It matters not how small the bore of the pipe may be ; the pres- 
sure depends solely on its height. 

320. Hydrostatic Press. — A useful application 
of the same principle is made in Bramah's Hydro- 
static (or Hydraulic) Press, exhibited in Fig. 152. 

E B represents a forcing-pump worked by the lever A. This 

instrument, which is fully described on page 188, consists of a 

piston working within a small tube to which it is tightly fitted, 

and which descends, as shown by the dotted lines, into a 

cistern in the bottom of the frame of the press. F G is a tube 

connect- 

Fig. 152. 




HYDB08TATI0 BELLOWS 



iron cylinder D, free to move up 
and down within it. D has a plat- 
en, H H, attached to it, between 
which and the top of the frame, 
the cotton, hay, cloth, or other 
substance to be pressed, is placed. 
To work the press, raise the 
long arm of the lever A. Water 
is by this means drawn up from 
the cistern into the tube E B ; and, 
when A is lowered and the piston 
thus made to descend, being pre- 
vented from returning to the cis- 
tern by a valve which closes, it is 
forced through the tube F Gr into 
the lower part of the cylinder C. 




HYDROSTATIC OB HYDBATJLIC PRESS. 



D being thus driven up and with it the 



platen, whatever is confined between the latter and the top of the frame b 



performed with it. How great a weight will it support ? 320. Describe the Hydrostatic 
Press, with the Figure. How is it worked ? How great pressure may be obtained 



SPECIFIC GRAVITY. 139 

subjected to pressure, greater or less according to the quantity of water 
forced into C. 

With the Hydrostatic Press any degree of pressure 
may be obtained that is not too great for the strength of 
the materials employed. The machine is extensively used, 
not only for pressing, but also for extracting stumps, test- 
ing cables, and raising vessels out of water. 

Specific Gravity. 

321. If we weigh a cubic inch of water, and then the 
same bulk, or volume, of silver, and of cork, we find the 
silver heavier than the water, and the cork lighter. If we 
proceed to compare the weights of various other substances, 
taking a cubic inch of each, we shall find that they all differ 
more or less. To express the comparative weight of differ- 
ent substances, the term Specific Gravity is used. 

322. The Specific Gravity of a substance is the weight 
of a given bulk of it compared with the weight of an equal 
bulk of some other substance taken as a standard. The 
standard employed is distilled water at the temperature of 
60 degrees. 

A standard of this kind must be invariable. Hence the temperature of 
the water is fixed; for at a higher degree of heat it would become rarer, — 
and at a lower degree, denser. Distilled water is taken, because it is pure ; 
the intermixture of vegetable and mineral matter in spring and river water 
affects their density, and makes them unfit for a standard. 

A cubic inch of silver weighs IOV2 times as much as a cubic inch of wa- 
ter; accordingly, the specific gravity of water being 1, that of silver is 10y 2 . 
A cubic inch of cork weighs 24 / 100 as much as the same bulk of water ; the 
specific gravity of cork, therefore, is set down at 24 / 100 or .24. 

323. Fluids that do not mix, when brought together, 
arrange themselves in the order of their specific gravities, 
the heaviest at the bottom. Thus, if mercury, water, and 
oil be thrown into a tumbler, the mercury will settle at the 



with the hydrostatic press ? For what is this machine used ? 321. If we weigh equal 
bulks of different substances, what do we find ? What term is used to express tho 
comparative weight of different substances ? 322. What is Specific Gravity ? What 
Is taken as a standard? Why is the temperature of the water fixed? Why is dis- 
tilled water taken ? What is the specific gravity of silver, and why? What is the 
specific gravity of cork, and why ? 323. How do fluids that do not mix, when brought 



1 4 HYDROSTATICS. 

bottom, because its specific gravity is greatest ; next will 
come the water ; and on top, the oil, which is the lightest 
of the three. 

Cream rises on milk, because its specific gravity is less than that of milk. 
For the same reason, the oily particles of soup float on the top. 

The negroes in the West Indies take advantage of this law of specific 
gravity. When they want to steal rum out of a cask, they introduce through 
the hole in its top the neck of a bottle filled with water. The water descends 
on account of its greater weight, and rum takes its place in the bottle. 

324. Gases, like liquids, differ in their specific gravity. Smoke rises, be- 
cause it is lighter than air. Hydrogen is so much inferior to air in specific 
gravity, that it not only rises itself, but also carries up a loaded balloon. 
Carbonic acid gas, on the other hand, is somewhat heavier than air ; it is 
therefore found at the bottom of wells and mines, where its poisonous prop- 
erties sometimes prove fatal to those who descend. 

325. If a solid floats on a liquid, like cork on water, its 
specific gravity is less than that of the liquid ; if it sinks, 
like lead, its specific gravity is greater. If solid and liquid 
have the same specific gravity, the solid will remain sta- 
tionary at any depth at which it is placed, without rising 
or sinking. 

That a solid may float, it is not essential that, in a compact mass, it weigh 
less than a like bulk of the liquid. A solid may therefore float or sink in 
the same liquid, according to the form it is made to assume. A cubic inch 
of iron weighs 7 J /4 times as much as a like bulk of water, and will therefore 
sink in the latter ; but, if beaten out into a vessel containing more than 7 l / 4 
cubic inches, this same iron will float, because then it is lighter than an equal 
bulk of water. It is on this principle that iron ships float. 

Fig. 153. 326. A floating solid displaces its own 

weight of liquid. 

To prove this, fill the vessel A with water up to the 
opening B. Drop in a ball of wood. As it becomes 
partially immersed, it raises the water and causes it to 
flow through B. Catch the water thus displaced, and 
it will be found to weigh exactly the same as the ball. 

327. A body immersed in water is 

together, arrange themselves ? Give an example. Why does cream rise on milk ? 
What use do the negroes in the West Indies make of this principle ? 324. What is 
said of the specific gravity of gases ? Why does smoke rise? Hoav does hydrogen 
compare with air in specific gravity ? Carhonic acid ? 325. When will a solid float 
on a liquid, when sink, and when remain stationary without rising or sinking ? How 
may a solid which in a compact mass is heavier than water, he made to float? 




SPECIFIC GRAVITY OF LIQUIDS. 



141 



buoyed up, and loses as much weight as the water it dis- 
places weighs. 

A boy can bring up from the bottom of a pond a heavy stone which he 
could not lift on land. In raising a bucket from a well, we find it become 
heavier the moment it leaves the water. In each case, the weight of the ob- 
ject, while in the water, is diminished by its upward pressure. 

That the weight thus lost equals Fi<r 154, 

that of the water displaced, is shown 
with the apparatus represented in Fig. 
154. From one side of a balance sus- 
pend a solid cylinder B, and on the same 
scale place a hollow cylinder A, which 
just contains the other. Balance the 
whole with a weight C in the opposite 
scale. If, now, we immerse B, still sus- 
pended, in a vessel of water, C will be 
found to outweigh AB, but the differ- 
ence is exactly made up by filling A with 
water; and as A just holds B, it is evi- 
dent that it holds as much water as B dis- 
places. 

328. Specific Gravity of Liquids. — The specific grav- 
ity of a body is simply its weight compared with that of a 
like bulk of water. Hence the specific gravity of a liquid 
may be easily obtained in the following way : Fill a glass 
vessel, whose weight is known, with water up to a certain 
mark, and weigh it ; subtract the weight of the vessel, and 
you have the weight of the water alone. Then fill the ves- 
sel to the same height with the liquid in question, weigh it 
again, and subtract the weight of the vessel as before. To 
find the specific gravity of the liquid, divide its weight by 
that of the water. 

A flask that will hold 1,000 grains of water, called the Thousand Grain 
Bottle, is often used for this purpose. A glass stopper, with a narrow open- 
ing running lengthwise through it, is fitted to the neck. The flask being 
filled, this stopper is inserted ; as it descends, it forces out the excess of 
liquid through its opening, and thus always ensures the same volume of liquid 




GHve an example. 326. How much liquid does a floating solid displace ? Prove this 
with Fig. 153. 327. How much weight does a body immersed in water lose ? Give 
pome familiar examples of this loss of weight. Prove, with the apparatus represented 
in Fig. 154, that the weight lost equals that of the water displaced. 328. How may 
the specific gravity of a liquid be obtained? What is the Thousand Grain Bottler 



142 



HYDROSTATICS. 



inside. A flask containing 1,000 grains of water will hold 13,568 grains of 
mercury and 792 grains of alcohol ; dividing according to the rule, we find 
the specific gravity of mercury to be 13.568, and that of alcohol .792. 

329. The Hydrometer. — The specific gravity of liquids 
may also be determined by the Hydrometer. This instru- 
ment consists of a hollow ball, C, from which 
rises a graduated scale, A ; while to its lower 
side is attached a solid ball, B, of sufficient 
weight to keep the instrument in a vertical po- 
sition. 

To find the specific gravity of any liquid, place the hydrom- 
eter in it. The rarer the liquid, the farther it descends ; and 
the figure on the scale at the point where it meets the surface, 
is noted. A table accompanies the instrument, which tells the 
specific gravity of a liquid when the height to which it rises 
on the scale is known. 

The hydrometer is used by dealers in spirits, oils, and 
chemicals, to test their strength. The height to which the 
pure article rises on the scale being known, any different re- 
sult when a liquid is tested, indicates adulteration. 

330. Specific Gravity or Solids. — The 
simplest way of finding the specific gravity of a 
solid would be to take a certain bulk of it (say a cubic inch 
or cubic foot), ascertain its weight, and divide it by the 
weight of a like bulk of water. It is so difficult, however, 
to obtain any given bulk exactly, that other methods have 
to be resorted to. 

331. If the solid sinks in water, weigh it first in air, and 
then in water by means of a balance prepared for the pur- 
pose. Divide its weight in air by the weight it loses in 
water, and the quotient will be its specific gravity. 

This is the same thing as dividing the weight of the solid by that of an 
equal bulk of water, for we have already seen that a solid weighed in a liquid 
loses as much weight as the liquid it displaces weighs. 




THE HYDROM- 
ETER. 



How many grains of mercury will such a flask hold ? Of alcohol ? What, then, is 
the specific gravity of mercury and alcohol ? 329. What instrument is used for ob- 
taining the specific gravity of liquids ? Describe the Hydrometer. How is the spe- 
cific gravity obtained with this instrument? By whom is the hydrometer cbieflj 
used? How does it indicate adulteration? 330. What would be the simplest mode 
of finding the specific gravity of a solid? What difficulty stands in the way? 
881. How may we find the specific gravity of a solid that sinks in water? Give an 



SPECIFIC GRAVITY OF SOLIDS. 143 

A piece of platinum weighs 22 grains in air, and 21 in water. Dividing 
22, the weight in air, by 1, the loss of weight in water, we get 22 for the spe- 
cific gravity of platinum. 

332. To find the specific gravity of a solid that floats on 
water, attach to it some body heavy enough to sink it. 
Weigh the two, thus attached, in air and in water ; and by 
subtraction find their loss of weight in water. In the same 
way, find how much weight the heavy body alone loses in 
water. Subtract this from the loss sustained by the two, 
and you get the weight of a volume of water equal to the 
body under examination. Divide the body's weight in air 
by this remainder, and you have its specific gravity. 

Example. Required the specific gravity of a piece of elm wood weighing 
2 ounces. Attach to it 4 ounces of lead. 

The combined solids weigh in air 2 + 4 = 6 ounces. 
In water we find them to weigh 3.15 ounces. 

Loss of the combined solids in water, 2.85 ounces. 

The lead alone weighs in air 4 ounces. 

The lead alone weighs in water 3.65 ounces. 

Loss of the lead in water, 35 ounce. 

Weight of a volume of water equal to the wood, 2.85 — .35 = 2.50 
Specific gravity of elm wood, 2 -r- 2.50 = .8 

333. Specific Gravity of Gases. — The specific gravity 
of gases is found by a process similar to that employed for 
liquids. Air is taken for the standard. A glass flask fur- 
nished with a stop-cock is weighed when full of air, and 
again when exhausted by means of an air-pump ; the differ- 
ence between these weights is the weight of a flask-full of 
air. The flask is then filled with the gas in question, and 
again weighed ; this weight, less that of the exhausted flask, 
is the weight of a flask-full of the gas. Divide the weight 
of the gas by that of the air, and the quotient is the spe- 
cific gravity required. 

334. Tables of Specific Gravities. — The following 



example. 332. How may we find the specific gravity of a solid that floats on water ? 
Find the specific gravity of a piece of elm wood weighing 2 ounces. 833. What is 
taken for a standard in estimating the specific gravity of gasce ? How may the spe 



144 



HYDROSTATICS. 



tables give the specific gravity of some of the most impor- 
tant substances : — 

Specific Gravity of Solids and Liquids. — Standard, Distilled Water, 1. 



Iridium 23.000 

Platinum... 22.069 

Gold 19.358 

Mercury .. .. 13.568 

Lead ... 11.445 

Silver 10.474 

Copper, cast 8.788 

Tin 7.291 



Iron, cast ...... 

The earth 

Diamond . = . 
Parian Marble . . 
Anthracite coal.. 
Bituminous coal. 
Lignum vitae. . . . 
Oak 



■7.207 
5.210 
3.536 
2.838 
1.800 
1.250 
1.333 
.970 



Ice 930 

Living men.. .891 

Cork... 240 

Human blood 1.045 

Milk 1.030 

Sea water . . . 1.026 

Olive oil 915 

Alcohol 792 



Specific Gravity of Gases. — Standard, Air, 1. 



Hydriodic Acid 4.300 

Carbonic Acid 1.524 

Oxygen . 1.111 



Air ..., 1.000 

Nitrogen 0.972 

Hydrogen 0.069 



335. By examining the above tables, it will be found that solids generally 
have a greater specific gravity than liquids, and liquids than gases. Among 
solids, the metals are the heaviest. 

The heaviest known substance is the metal iridium, which, bulk for bulk, 
weighs 23 times as much as water. The lightest substance is hydrogen gas. 
It would take about 12,000 cubic feet of hydrogen to weigh as much as one 
cubic foot of water. 

Sea-water, being impregnated with salts, is somewhat heavier than fresh 
water. It is therefore more buoyant ; and this every swimmer that has tried 
it knows. A vessel passing from fresh water to the sea, draws less water in 
the latter, that is, does not sink to so great a depth. 

336. Water is 81 5 times heavier than air ; that is, it would take 815 cubic 
inches of air to weigh as much as 1 cubic inch of water. Hence, by confin- 
ing air in tight chambers in different parts of life-boats, they are made so 
buoyant that they can not sink even when filled with water. Life-preservers 
act on the same principle. The air confined in them, being 815 times lighter 



cific gravity of gases be found ? 334 [Questions on the Tables.— "Which is the densest 
of the metals ? Which is the densest of liquids ? "Will the wood called lignum vitce 
float in water ? "What liquid will it float in ? "Which weighs more, a cubic foot of 
water or the same bulk of ice ? In which would a boat sink deepest, olive oil, alco- 
hol, or sea-water ? Could a man swim in alcohol ? "Would a balloon rise most easily 
In hydrogen, carbonic acid, or air? "Would a balloon filled with oxygen rise in air?] 
835. How do solids, as a general thing, compare with liquids in specific gravity ? How 
do gases compare with liquids ? Among solids, what class of bodies are heaviest? 
What is the heaviest known substance ? How does its weight compare with that of 
water? "What is the lightest substance? How many cubic feet of hydrogen would 
it take to weigh as much as one cubic foot of water? How does sea-water compare 
with fresh water in specific gravity ? In which is it easier to swim ? In which does 
a vessel draw less water ? 336. How does air compare with water in specific gravity? 



SPECIFIC GRAVITY. 145 

than the same bulk of water, helps to keep up the bodies to which they may 
be attached. Many species of fish are provided with bladders, which they 
can fill with air or exhaust at pleasure ; they are thus able to increase or di- 
minish their specific gravity instantaneously, and to rise or sink accordingly. 
337. The specific gravity of living men is set down at .891, or less than » ,„ 
of that of water. The human body, therefore, will float; and, if the head is 
thrown back so as to bring the mouth uppermost, there is no danger of drown- 
ing, even in the case of those who can not swim. If the air is expelled from 
the lungs, and water takes its place, the specific gravity is increased ; conse- 
quently the bodies of drowned persons sink. After remaining under water 
for a time, they again float ; this is owing to the generation of light gases 
within them, by which their specific gravity is lessened. 

338. If we know the specific gravity of a body, we can 
easily find how much any given bulk of it weighs. A cubic- 
foot of water is found to weigh 1,000 ounces, or 62£ pounds 
avoirdupois ; the weight of a cubic foot of any given sub- 
stance will, therefore, be equal to 62^ pounds multiplied by 
its specific gravity. 

Exam-pie. Required the weight of a cubic foot of gold. The table makes 
the specific gravity of gold 19.358. Multiplying this into 62.5, we get 
1209.875 pounds for the weight required. 

339. Two solids of equal bulk will displace equal quan- 
tities of a liquid in which they are immersed ; but two sol- 
ids of equal weight will not do so, unless their specific grav- 
ity is the same. This principle has been applied in testing 
the purity of the precious metals. 

If, for instance, we wish to find whether a piece of silver is pure, we put 
it in a vessel even full of water, and catch what overflows : we do the same 
with an equal weight of what is known to be pure silver. If equal quantities 
of water are displaced, the article tested is pure, for it has the same specific 
gravity as pure silver ; but if not, it is adulterated. 

340. The fact above stated was discovered and first applied by Archime- 
des. Hiero, king of Syracuse, having purchased a golden crown, and sus- 
pecting the purity of the metal, asked the philosopher to test it, without in- 
jury to its costly workmanship. In vain Archimedes tried to solve the prob- 

On what principle are life-boats and life-preservers constructed ? How are fish ena- 
bled to rise or sink at pleasure ? 337. How does the body of a living man comparfc 
with water in specific gravity ? What follows, as regards danger of drowning ?• Why 
do the bodies of drowned persons at first sink, and afterwards rise ? 33S. If we know 
the specific gravity of a body, how may we find the weight of any given bulk of it ? 
Give an example. 339. When will two solids immersed in a liquid displace equal 
quantities? To what has this principle been applied? How, for example, may wo 
find whether a piece of silver is pure ? 340. By whom was this principle discovered ? 

10 



146 H YDROST ATIOS. 

lem ; till one day, when bathing, he observed, that, as more and more of his 
body became submerged, the water rose proportionally higher and higher in 
the vessp' . It at once occurred to him that any body of equal weight and 
exactl; , ■.■,■■ same density, but no other, would cause an equal rise of the liquid ; 
and here was a clue to the solution of the problem that had troubled him. 
Naked as he was, he rushed home from the bath, shouting u Heureka ! " J 
have found it I He immediately procured a quantity of pure gold equal in 
weight to the crown, and a like weight of pure silver. Then successively 
plunging the gold, the silver, and the crown, in a vessel brim-full of water, 
he caught and weighed the liquid displaced in each case. Finding that the 
crown displaced more than the gold and less than the silver, he inferred that 
it was neither pure gold nor pure silver, but a mixture of the two. Archi- 
medes afterwards investigated the subject further, and discovered the leading 
principles connected with specific gravity. 

Capillary Attraction. 

341. If one end of a fine glass tube be placed in a ves- 
sel of water, the other end being left open, the water will 
rise in the tube above its level. The force that causes the 
water to rise is known as Capillary Attraction. It is so 
called from the Latin word capillus, a hair, because it is 
most striking] y exhibited in tubes as tine as a hair. 

A liquid will not rise by capillary attraction in tubes 
that exceed one-tenth of an inch in diameter. 

342. A liquid will rise in a capillary tube, when the 
attraction of the solid for the liquid is more than half that 
of the liquid particles for each other. In such cases, the 
liquid will wet the solid; and the surface of the liquid will 
be concave, being raised where it touches the tube. 

The same thing is seen when a 
glass plate, C, is placed perpendicu- 
larly in water, A B : the surface, in- 
stead of maintaining the same level 

throughout, rises near the glass on 

both sides, as represented by the 
dotted lines. 

This experiment shows that th« 
attraction of glass for water is sufficiently great to overcome the gravity of 

Relate the circumstances. 341. What is Capillary Attraction ? Why is it so called ? 
What is the limit of size for capillary tubes ? 342. In what case will a liquid rise in 
a capillary tube ? What form of surface will the liquid present ? When a glass plate is 
placed perpendicularly in water, what maybe observed ? What does this experiment 



Fig. 156. 
C 



CAPILLARY ATTE ACTION. 147 

the latter. The adhesion "between glass and water, moreover, is greater 
than the cohesion between the particles of water ; for, if the glass be re- 
moved, some of the liquid will adhere to its surface,— that is, it will be wet. 

343. If the attraction of the solid for the liqu* i less 
than half that of the liquid particles for each other, the 
liquid will not rise in the tube, and its surface will become 
convex, being depressed where it touches the tube. 

In like manner, if the glass plate in the last experiment he" greased, 
the water, instead of being elevated near the sides, will he depressed, as 
shown by the dotted lines in Fig. 

157. A similar appearance is pre- ^ ig ' 15 ^* 

eented when a glass plate is plunged -F 

into mercury. In such cases, the 

liquid does not wet the solid ; when jf - 

the glass plate is drawn out, not a 
particle of the mercury adheres to 
it. 

This apparent repulsion may he 
so great as to prevent a solid from sinking in a liquid lighter than itself. 
A fine needle smeared with grease, if carefully laid in a horizontal position 
on the surface of still water, will remain floating there. It is thus that 
insects are able to walk on water, their feet not sinking in the liquid or 
even becoming wet. 

344. Familiar Examples. — Examples of capillary at- 
traction meet us on all sides. 

If one end of a towel be left in a basin of water, the 
part outside soon becomes wet, the liquid being drawn up 
through its minute fibres. The same thing happens if a 
piece of sponge, of bread, or of sugar, remains in contact 
with a liquid, the pores of the substance acting like capil- 
lary tubes. Blotting paper drinks up ink on the same 
principle. 

The common lamp affords a good illustration of capillary attraction. The 
oil or burning-fluid is drawn up through the fibres of the wick fast enough 
to support the flame. There is a limit, however, beyond which capillary at- 
traction does not act ; and, therefore, if the oil gets low, the lamp grows 
dim and finally goes out. To allow a free passage to the oil, the little tubes 

■show? 343. Under what circumstances will an apparent repulsion be manifested 
between the solid and liquid? What form of surface will the liquid in the tube then 
present? Give examples. What is sometimes the consequence of this apparent re- 
pulsion ? Give an example. How is it that insects walk on water ? 344. How is capil- 
lary attraction illustrated with a towel and a piece of bread or sugar? How is the flame 



148 



HYDROSTATICS. 



must be kept clear ; and, as impurities gather in them from the ascending 
liquid, the wick must be changed from time to time. 

Capillary attraction is strikingly exhibited in wood. Water is drawn up 
into its pores, distending them, and causing a perceptible increase of size. 
This expansion is turned to practical account in the south of France. A 
large cylinder of free-stone, several feet in length, has circular grooves made 
at intervals in its surface. Into these grooves are driven wedges of dry 
wood, which are then kept wet with water. As the wood absorbs the liquid, 
it gradually expands, till it rends the solid cylinder into rough mill-stones, 
which require but little labor to fit them for market. 

It is capillary attraction that renders the banks of streams so productive ; 
the water drawn in through the pores of the earth, fertilizes the adjacent 
parts. On the same principle, a potted plant may be supplied with the ne- 
cessary moisture by filling the saucer in which it stands with water. Houses 
are rendered damp by the absorption of external moisture, the pores of the 
brick or stone, of which the walls are built, acting as capillary tubes. 

345. Laws of Capillary Attraction. — Different li- 
quids rise to different heights in tubes of the same size. 
Ether, for example, rises about one-half, and sulphuric acid 
only one-third, as high as water. 

The same liquid always rises to the same height in a 
tube of given size ; and this height is proportioned to the 
fineness of the bore. In a tube T ^ ¥ of an inch in diameter. 



water rises 5 T \ inches. 

Fig. 158. 



Fig. 159. 





346. Fig. 158 represents six tubes of 
different bore, communicating at the bot- 
tom with a vessel containing colored wa- 
ter. The water rises according to the fineness of the bore, standing highest 
in the smallest tube. 



of a lamp supplied with fuel '? How is capillary attraction exhibited in wood ? "What 
use is made of this principle in France ? What is the effect of capillary attraction on 
the banks of streams ? How may a potted plant be supplied with moisture ? How 
are houses made damp? 345. What is the law of capillary attraction, as regards dit 



CAPILLARY ATTRACTION. 



140 



S47. The same principle is illustrated with two glass plates (see Fig. 159), 
joined at one end and slightly diverging so as to form an angle of about 
two degrees. Let the plates rest in colored water to the depth of an inch, and 
the liquid will rise between them, reaching the greatest height where the 
surfaces are nearest together, and thus forming the curve called the hy-per'- 
bo-la, 

348. Interesting Facts. — If a capillary tube capable 
of raising water four inches be broken off at three, there 
will be no overflow, as might be expected. The water will 
rise three inches to the top of the tube, and there stop. 
But it will be supplied as fast as evaporation takes place. 
Hence, to prevent waste in a spirit lamp, an extinguisher 
is put over the wick when it is not burning. 

It is a remarkable fact that no evaporation takes place unless the liquid 
reaches the top of the capillary tube. Tubes containing as much water as they 
could hold under the influence of capillary attraction, have been hung in the 
sun for months, without losing any part of their contents by evaporation. 

349. Floating Bodies. — Motion is produced in bodies 
floating near each other, by a force resembling capillary 
attraction. This may be shown with two balls, as repre- 
sented in Figs. 160, 161, 162. 

A and B are cork balls, capable of being wet 
with water. When they are brought close to- 
gether, the attraction of their surfaces raises the 
water around them ; the column that separates 
them becomes thinner and thinner, till at last 
they touch. 

C and D are similar balls, greased so that 
they can not be wet. In this case, the surface 
of the surrounding water is repelled, forming 
little hollows in which they rest. Since there 
is not enough liquid between them to balance 
the pressure from without, the balb again ap- 
proach each other. 




Fig. 161. 




ferent liquids ? Give an example. Whj.t is the law for the same liquid in a tube of 
given size ? How high does water rise in atube Vioo of au inch in diameter? 346. What 
does Fig. 158 represent? 347. Describe the experiment with two glass plates. 
848. What fact is stated respecting a capillary tube broken off at the top ? Why is it 
necessary to put an extinguisher on a spirit-lamp ? What fact is stated respecting 
evaporation from capillary tubes ? 349. How are floating bodies affected by a force 
resembling capillary attraction ? What, for example, is the effect on cork balls capa- 
ble of being wet? On balls "Teased so that thev can not be wet? On balls, one of 




150 HYDROSTATICS. 

E and F are a pair of similar balls, one of 
which, E, can be wet, while the other, F, can 
not. The water, attracted by E, rises around it, 
whereas around F it is depressed. If these balls 
are placed near together, F, being repelled from 
the wall of water around E, will recede from it. 

350. Endosmose and Exosmose. — Two peculiar results 
of capillary attraction, known as Endosmose and Exosmose, 
remain to be mentioned. 

Endosmose is the inward motion of a fluid, through a 
membranous or porous substance, into a vessel containing 
a different fluid. Exosmose is the outward motion of the 
contained fluid through the same substance. 

Fill a vessel with alcohol, tie over the top a bladder that has been soaked 
in water, and immerse the whole in water. In a few hours it will be found 
that water has passed into the vessel through the bladder, and that alcohol 
has passed out into the water. The former movement is called Endosmose ; 
the latter, Exosmose. The inward current is stronger than the outward one. 
Water passes in faster than alcohol escapes ; and consequently the bladder 
soon becomes puffed out. All membranous and porous substances, such as 
india rubber, plaster of paris, wood, &c, permit the passage of these cur- 
rents, which are owing to capillary attraction. 

351. Endosmose and exosmose are exhibited in the case 
of gases, as well as liquids. 

If a phial full of air, with a piece of thin bladder tied over its mouth, be 
placed in a jar of carbonic acid gas, the latter will force its way into the 
phial while air will pass out. Here, again, the inward current is the stronger ; 
the bladder is puffed out, and finally bursts. 

The facility with which gases thus pass in and out through porous sub- 
stances is proportioned to their rarity. Hydrogen, the rarest of known 
bodies, exhibits these movements in their greatest perfection. This is the 
reason why the rose balloons, recently so popular as toys, lose their buoy- 
ancy in a few days. They are made of thin india rubber, and filled with hy- 
drogen. When allowed to remain in the air, endosmose and exosmose take 
place. Hydrogen passes out through the pores of the rubber, and air takes 
its place. The balloon gradually becomes less buoyant, ceases to rise, and at 
last, as it loses more of its hydrogen, is carried to the ground by the weight 
of the india rubber. 



which can be wet and the other not? 350. What is Endosmose? What is Exosmose? 
Show how endosmose and exosmose operate. Through what sort of substances do 
they take place? 351. What, besides liquids, are affected by these movements? 
Give an example. What gases most readily pass in and out through porous sub- 
stances ? What gas exhibits endosmose and exosmose most distinctly ? What is the 



EXAMPLES FOR PRACTICE. 151 

352. The skin being porous, a liquid with which it re- 
mains in contact will find its way through by endosmose 
and be absorbed by the body. If a drop of the powerful 
poison called prussic acid be placed on the arm, a suffi- 
cient quantity to cause death will thus be taken into the 
system. 

353. Endosmose and exosmose enter largely into tho 
operations of nature. They cause the ascent and descent 
of sap in trees and vines. The inside of living plants con- 
sists of minute cells, containing fluids of different densities. 
These fluids are constantly passing in and out through the 
porous walls which separate them, under the influence of 
exosmose and endosmose, modified by the vital action at 
the same time going on. 

EXAMPLES FOR PRACTICE. 

1. (See §328.) A phial weighing 4 ounces when empty, weighs 6 ounces when 

filled with water, and 7 when filled with nitric acid. Required, the spe- 
cific gravity of the acid. — Ans. 1.5. 

2. A vessel filled with ether weighs 13.575 ounces ; filled with water, 15 

ounces ; when empty, 10 ounces. What is the specific gravity of ether ? 

3. An empty jar weighs 7.5 pounds ; filled with sulphuric acid, it weighs 

12.1125 pounds ; and filled with water, 10 pounds. Find the specific 
gravity of sulphuric acid. 

4. A Thousand Grain Bottle is found to hold 870 grains of oil of turpentine, 

and 1,036 grains of oil of cloves. What is the specific gravity of these 
oils? 
In which would a cork ball sink the deeper ? 

5. (See §331.) A piece of crown-glass weighs 5 ounces in the air, and 3 in 

water. What is its specific gravity ? — Ans. 2.5. 

6. A beef-bone weighs 2.6 ounces in water, and 6.6 ounces in air. What is 

its specific gravity? — Ans. 1.65. 

7. What is the specific gravity of a piece of ivory, which weighs 16 ounces 

in air, and loses 8% ounces when weighed in water? — Ans. 1.828 + 

8. ( To solve the next two problems, see § 332 and Example. In each case, we 

may suppose a pound (16 ounces) of lead, weighing 14.6 ounces in water, 
to be used for sinking the solid.) 
A piece of wax weighs 8 ounces ; when it is fastened to a pound of 

effect of these movements on rose balloons ? 352. What is their effect, when a liquid 
is placed on the skin ? Give an example. 853. Whai is the effect of endosmose and 
exosmose in trees and vines ? 



152 HYDRAULICS. 

lead, the whole weighs in water 13.712 ounces. What is the specific 
gravity of the wax? — Ans. .9. 

9. Fastening a piece of ash to a pound of lead, I find their weight in water 

to be 12.76 ounces. The ash alone weighs 10 ounces in the air. What 
is its specific gravity? — Ans. -84 + 

10. (See § 333.) A glass flask, with the air exhausted, weighs 4 ounces ; 
filled with air, it weighs 4.25 ounces; and filled with cy-an'-o-gen, 
4.45125 oz. What is the specific gravity of cyanogen? — Ans. 1.805. 

.1. A flask full of chlorine weighs 11.222 ounces. Filled with air, it weighs 
10.5 oz., and when the air is drawn out, 10 oz. Eequired, the specific 
gravity of chlorine. 

12. According to the answers of the last two questions, in which would a 
balloon rise most easily, air, cyanogen, or chlorine ? 

13. (See % 336.) How many cubic feet of air would it take to weigh as much 
as 4 cubic feet of water ? — Ans. 3,260 cubic feet. 

14. (See % 338, and Table.) How much would a cubic foot of gold weigh? 
How much, the same bulk of silver ? 

15. What would be the weight of 4 cubic feet of Parian marble ? 

1 6. What is the weight of a block of anthracite coal, 6 feet long, 4 feet wide, 
and 3 feet high ? ( To find the number of cubic feet in the block, multiply 
the length, breadth, and thickness together.') — Ans. 8,100 pounds. 

17. Suppose a room 10 feet high, long, and wide, to be filled with gold, 
what would the gold weigh? — Ans. 1, 209, 875 pounds. 



CHAPTER XI. 

MECHANICS (CONTINUED). 

HYDRAULICS. 

354. Hydraulics treats of liquids in motion, whether 
issuing from orifices or running in pipes and the beds ol 
streams. It shows how water is applied as a moving power, 
and describes the machines used for raising liquids. 

355. Flow or liquids through orifices. — If an orifice 
be made in the side or bottom of a vessel containing a liquid, 
the latter will escape through it. The particles of liquid 
near the orifice are forced out by the pressure of those 
above. 



FLOW OF LIQUIDS THROUGH ORIFICES. 153 

356. Velocity. — The velocity of a stream flowing through 
an orifice depends on the distance of the latter below the 
surface of the liquid, being equal to the velocity which a 
body would acquire in falling that distance. 

If, for instance, in a reservoir full of water, three orifices be made at 
depths of l^jn, 64^3, and 144 3 /4 feet, the liquid (leaving friction, &c, out of 
account) will issue from them with velocities of 32y 6 , 647 3 , and 96V2 f eet per 
second, because such, as we have found, would be the velocity of a body fall- 
ing through the different distances first named. 

The distances above mentioned are to each other as 1, 4, 9 ; the velocities 
are to each other as the square roots of these numbers,- 1, 2, 3. Consequent- 
ly, the velocities of streams issuing from different orifices in the same vessel 
are to each other as the square roots of their respective distances below the sur- 
face of the liquid. Friction, however, and other causes, produce more or 
less deviation from this rule. 

357. As long as the liquid is kept at the same height in 
the vessel, it issues from a given orifice with the same ve- 
locity ; but, if the vessel is not replenished, as the liquid 
gets lower, the pressure diminishes, and the velocity of the 
stream diminishes with it. It takes twice as long to empty 
an unreplenished vessel through a given orifice, as it would 
for the same quantity of water to .escape if the liquid were 
kept at its original level. 

358. The Clepsydra. — Among the ancients, time was 
measured by the flow of water through an orifice, in an in- 
strument called the Clepsydra, or Water-clock. It consist- 
ed of a transparent vessel with a hole in the bottom that 
would empty it in a certain time. A scale on the side of 
the vessel indicated, by figures at different levels, the num- 
ber of hours which it took the liquid to reach them suc- 
cessively in its descent. As the discharge was most rapid 
when the vessel was full, the divisions were of course longest 
at the top of the scale. 

The clepsydra was necessarily inaccurate, inasmuch as the flow of the 



354. Of what does Hydraulics treat? 355. What causes a liquid to flow through 
an orifice in the vessel containing it ? 356. On what does the velocity with which a 
stream issues from an orifice depend? Give an example. What is the law for the 
velocities of streams issuing from different orifices in the same vessel ? 357. What 
difference does it make, as regards the velocity of a stream through an orifice, whether 
the vessel is kept replenished or not? 358. What did the ancients use for measuring 



154 HYDRAULICS. 

water varied in rapidity according to its temperature and the density of the 
atmosphere. Yet it answered for general purposes ; indeed, it was the only 
instrument used for measuring small intervals of time in astronomical ob- 
servations. 

359. Course of Streams flowing from Orifices. — A 
liquid issuing from an orifice descends in the same line as 
a projectile (see § 127). The curve described is called a 
parabola. In a given vessel, a stream will spout to the 
greatest horizontal distance, from an orifice midway be- 
tween the surface and the bottom of the liquid. Streams 
flowing through orifices equally removed from this central 
one, will spout to the same distance. 

Fig. 168. * n ^§- 1 ^ 3 ' *f tne ormce B be midway between 

the surface and the bottom of the liquid, the stream 
passing through it will spout to the greatest dis- 
tance ; and if A and C be equi-distant from B, the 
streams passing through them will reach the same 
point. 

360. Volume discharged. — To find 
the volume of liquid discharged in a 
given time from an orifice in a vessel 
that is kept replenished, multiply the area of the orifice by 
the velocity of the stream per second, and this product by 
the number of seconds. 

No allowance is here made for friction ; in practice, 
therefore, the discharge is less than would appear from 
this rule. 

Example. How much water will be discharged from an orifice of 2 square 
inches in 5 seconds, the velocity of the stream being 10 inches in a second, 
and the vessel being kept replenished ? — Ans. 2X10X5 = 100 cubic inches. 

361. The quantity discharged through a given orifice 
in a given time differs in the case of different liquids. Al- 
cohol, for instance, 'flows more slowly than water, and mer- 

rime ? Describe the clepsydra. What rendered the clepsydra inaccurate ? 359. What 
curve does a stream issuing from an orifice describe ? At what part of a vessel will a 
stream from an orifice spout to the greatest distance ? What is said of streams equal- 
ly removed from the central one? Exemplify these principles with Fig. 163. 
860. What is the rule for finding the volume of liquid discharged from an orifice in a 
given time ? What causes deviations from this rule in practice ? Give an example. 
361. What is said of the quantity discharged in the case of different liquids ? Give an 




FLOW OF LIQUIDS THROUGH ORIFICES. 



155 



cuiy more rapidly ; the discharge of alcohol will therefore 
be less, and that of mercury greater, than the discharge 
of water. 

362. A circular orifice of given area discharges more 
liquid in a given time than one of any other shape. This is 
because a circle is the smallest line that can enclose a given 
space ; in passing through a circular orifice, therefore, the 
liquid comes in contact with a less extent of solid surface, 
and is less retarded by friction. 

363. The volume discharged through an orifice in a given time may be 
increased by heating the liquid. Heat lessens its cohesion, and enables it to 
flow more rapidly. 

364. The discharge may also be increased by fitting a short tube, or Ad- 
jutage, to the orifice. The minute currents of the particles are thus prevent- 
ed from obstructing each other in the act of passing out. The best shape for 
such a tube is that of a bell with the large „ p. 16 ~ F - -^g 



W W 



"ge, it obstructs 



end out, as shown at A in Fig. 164. When 
such a tube is used, the discharge in a given 
time is increased one-half ; and there is a 
still greater gain, if the bottom of the ves- 
sel is rounded to meet the tube, as at B in 
Fig. 165. 

If, however, the tube extends into the 
vessel, as at C in Fig. 166, instead of increasing the disch 
and diminishes it. 

365. Flow of liquids in pipes and the beds of 
streams. — The friction of water against the sides of pipes 
in which it is conveyed, retards its velocity and diminishes 
the quantity discharged. 

When the distance is great, or there are sudden turn- 
ings, allowance must be made for friction by increasing the 
size of the pipes, or the quantity discharged will fall far 
below what is required. If, for instance, leaving friction 
out of account, pipes 6 inches in diameter would yield the 
desired supply, nine-inch pipes would be none too large to 
use. 



example. 362. With a given area, what shape must an orifice have, to discharge the 
most liquid ? Why is this ? 363. How may the volume discharged be increased ? 
864. What other mode of increasing the discharge is there ? Describe the kinds of 
Adjutage mentioned in the text, and state the effect of each. 365. What is the effect 
of friction on the flow of liquids ? How great an allowance should be made for fric- 



156 HYDRAULICS. 

366. Rivers. — The friction of a stream against its banks 
and bottom materially retards its motion. Hence the ve- 
locity of a river is always less near its banks than towards 
the centre, and near the bottom than at the surface. 

The windings of a stream also lessen its velocity. Were 
it not for their numerous bends, many large rivers would 
flow so rapidly that they could not be navigated. 

367. The velocity of a stream depends much on the slope of its bed. A 
river with but few bends, and a fall of three inches to the mile, moves at the 
rate of about three miles an hour. As the slope increases, the velocity rap- 
idly increases also ; and a fall of three feet in a mile gives the impetuosity ol 
a torrent. 

Sometimes the bed of a river has a considerable fall at first, and then be- 
comes comparatively level. In such cases, the impetus of the water keeps 
it in motion at a rate proportioned to its volume. The fall of the Amazon, 
in the last 700 miles of its course, is only 12 feet. 

368. The quantity of water discharged by a stream de- 
pends on its size and velocity. In large rivers, it is almost 
incredible. The discharge of the Mississippi is estimated 
at twelve billions of cubic feet every minute, and that of 
the Amazon is nearly four times as great. 

369. Waves. — Waves are caused by the action of wind 
on a liquid surface. As the particles of a liquid move freely 
among each other, the undulations produced directly by 
the wind extend along the surface to a great distance, far- 
ther than the wind itself. 

The wind is enabled to take hold, as it were, of the water, and produce 
waves, by the friction at the surface. This friction may be diminished, just 
as in the case of machinery, by covering the surface with oil. The wind 
then slips over it, and the water becomes comparatively calm. It is said 
that boats have been enabled to get through a dangerous surf in safety, by 
emptying barrels of oil upon it. 

370. Waves appear to move forward, but in deep water they only move 



tion ? 366. Where has the water of a river the least velocity, and why ? What effect 
iiave the windings of a stream on its velocity ? 367. On what does the velocity of a 
Stream chiefly depend ? How great a velocity does a fall of three inches in a mile 
prodnce ? How great a fall produces the velocity of a torrent ? How great a fall has 
the bed of the Amazon near its mouth ? What keeps its waters in motion ? 36S. On 
what does the quantity of water discharged by a stream depend ? How great is the 
discharge of the Mississippi? Of the Amazon? 369. By what are waves caused? 
What enables the wind to produce waves? How may a rough sea be calmed? 



WAVES. 157 

op and down. A floating body, after rising and falling with successive waves, 
when the sea becomes calm is found in the same spot as before. If, how- 
ever, shoals or rocks interfere with the undulations, an onward motion is 
produced, and breakers are formed. Waves are always found breaking on a 
rocky shore, whatever way the wind may blow. 

371. Waves do not generally exceed 20 feet in height, 
— that is, do not rise more than 10 feet above, and sink 
more than 10 feet below, the usual level of the sea. They 
sometimes, however, attain a height of 40 feet. Vast and 
mighty as they are, their effects are confined to the surface, 
never extending to the great body of the ocean. The se- 
verest gales are not felt at a depth of 200 feet. 

372. Tides. — In the ocean, and the bays, rivers, <fcc, 
communicating with it, there is an alternate rise and fall 
of water, each lasting about six hours. These movements 
are called Tides. When rising, the tide is said to flow ; 
when falling, to ebb. 

373. Tides are caused chiefly by the attraction of the moon. This body, 
when opposite any given part of the earth, attracts the water at that part 
most strongly towards itself, and causes high tide. At the same time it is 
high tide at the opposite point of the globe, because the moon, attracting the 
mass of the earth more strongly than the more distant water on its surface, 
draws the former, as it were, away from the latter. These elevations are 
accompanied with corresponding depressions, or low tides, at other points. 

The sun, also, attracts the water on the earth's surface ; but not so strongly 
as the moon, in consequence of its vast distance. When sun and moon act 
in the same line, which happens at every new and full moon, the tides are 
highest, and are called Spring-tides. When sun and moon act at right 
angles, the tides are lowest, and are called .Neap-tides. 

374. The height of the tide is affected by prevailing 
winds, the shape of adjacent coasts, and other circum- 
stances ; accordingly, it is different in different places. At 
•St. Helena, the rise of water is only 3 feet ; in parts of the 
British Channel, it is 60. The highest tides known are in 
the Bay of Fundy, where they attain a height of 70 feet. 



870. How do waves appear to move? How do they really move? What proof is 
there of this ? What is the effect of shoals or rocks ? 871. What is the height oi 
waves ? How far below the surface do they extend ? 372. What are Tides ? 873. B> 
what are tides caused ? What, besides the moon, attracts the water ? What are 
Spring-tides, and how are they caused ? What are Neap-tides, and when do they 
jccur ? 874. By what circumstances is the height of tides affeeted ? How great is 



158 



HYDRAULICS. 



This makes the average rise one foot every five minutes, — 
so rapid a flow that animals feeding on the shore are some- 
times overtaken and drowned. 

375. Water-wheels. — Running water is exceedingly 
useful as a moving power. Made to act on wheels, it causes 
them to revolve by its momentum, turns the shafts or axles 
connected with them, and thus sets machinery of various 
kinds in motion. 

The wheels moved by water-power are of four kinds ; 
the Undershot, the Overshot, the Breast-wheel, and the 
Turbine. 

376. The Undershot Wheel 
is represented in Fig. 167. A 
wheel, A B, attached to an axle, 
0, has a number of float-boards, 
c, d, e,f, fitted into its rim, at 
right angles, and at equal dis- 
tances from each other. The 
whole is hung in such a way that 
the lowest float-board, c, is im- 
mersed in running water, M N. 
The current, striking against 
several float-boards, which are 
more or less submerged, carries 
the wheel around. 

The stream is often conduct- 
ed to the wheel through a nar- 
row passage called a Race ; and 
its power is sometimes increased by giving the race a slight inclination (see 
Figure). In other cases, the water is made to strike the wheel immediately 
after issuing from the bottom of a dam, with a high degree of velocity pro- 
duced by the pressure of a large body of water. Yet, under the most favor- 
able circumstances, as the weight of the water does not act on the wheel, but 
only the force of the current, no more than one-fourth of the moving power 
can be made available. 

377. The Overshot Wheel is represented in Fig. 168. It consists of a 
wheel, A B, attached to an axle, 0, and having a number of buckets, c, d, e,f, 
on its rim, at equal distances. A stream is conducted through a race, G H, 




THE UNDERSHOT WHEEL. 



the rise at St. Helena ? In the British Channel ? In the Bay of Fundy ? 375. How 
is running water turned to account? Xame the four kinds of water-wheels. 376. De- 
scribe the Undershot Wheel. How is the stream often conducted to the wheel ? 
How is its power increased ? In other cases, how is a high degree of velocity pro- 
duced? How much of the moving power can be made available with this wheel? 



WATER-WHEELS. 



159 



Fig. 168. 




THE OVERSHOT WHEEL. 



and made to fall on 
the wheel from above. 
The weight of the wa- 
ter and the force with 
which it descends 
cause the wheel to re- 
volve. Another buck- 
et is brought under 
the stream, which in 
its turn is filled, and a 
new one is presented. 

As the wheel turns, 
the descending buck- 
ets gradually lose their 
water, so that by the 
time they commence 
rising they are entire- 
ly empty. As the de- 
scending buckets contain more or less water and the ascending ones contain 
none, the wheel is kept revolving ; and the weight of the stream, as well as 
its velocity, being turned to account, three-fourths of the moving power is 
saved. 

378. In the Breast- Fig. 169. 

wheel, shown in Fig. 169, 
there is a similar arrange- 
ment of apartments on the 
rim. The water is received 
halfway up, or still higher 
in the High Breast-wheel 
commonly used in this 
country ; and its weight is 
thus made available. This 
wheel ranks between the 
Overshot and the Under- 
shot in efficiency, saving 
three-fifths of the moving 



power. 

379. The Turbine, a 




THE BREAST-WHEEL. 



section of which is represented in Fig. 170, instead of being vertical, like the 
wheels just described, is horizontal. It consists of a wheel, AB, divided into 
a number of apartments, c, d, e,f, by curved partitions. To the inner rim 
of the wheel is fitted a fixed cylinder, Gr H, divided into apartments corre- 
sponding with those of the wheel, but running in the opposite direction. 

377. Describe the Overshot Wheel. Explain its operation. How much of the moving 
power does it utilize ? 378. In the Breast-wheel, how is the water received ? How 
much of the moving power is utilized? 379. Describe the Turbine. Explain it* 



160 



HYDEAULICS. 




THE TURBINE. 



Fig- 170. This fixed cylinder is connected with the 

base of an upright tube, J K, through the 
middle of which runs another tube, I. 

The water which is to set the machinery 
in motion enters JK, runs through the 
apartments of GH, is discharged by them 
into the corresponding apartments of the 
wheel, and passes out into a course pro- 
vided for its escape. It strikes the parti- 
tions nearly at right angles, and with great 
force in consequence of the pressure of 
the liquid in the tube. The wheel is thus 
made to revolve ; and a shaft connected 
with it from below and passing through 
the inner tube I, communicates the motion to machinery above. Wherever 
there is a fall of water, turbines are found very useful. They have been 
known to utilize, or turn to account, four-fifths of the motive power, — more 
than is saved by any other wheel. 

380. Propulsion of Boats. — The wheels of steamboats 
are not turned by running water, like those described above, 
but by machinery worked by steam. As they strike the 
water, the latter reacts on them ; and the boats are forced 
forward or backward, according to the direction in which 
their wheels turn. Paddles on their rim enable the wheels 
to strike the water more forcibly. 

As the paddles descend and ascend, they have to over- 
come a considerable resistance in a vertical direction, which 
retards their motion ; it is only when they are vertical in 
the water that their full effect is felt. The rolling of the 
boat, also, often interferes with their action, burying them 
too deep or raising them entirely out of water. These dis- 
advantages have led some to prefer a submerged screw to 
the paddle-wheel. The screw is placed in the stern ; and 
vessels moved by its means are called Screw Propellers. 

381. The resistance which a vessel encounters in passing 
through water depends on its shape. The narrower the 
vessel and the sharper its prow, the more readily it pene- 



operation. TTow much of the moving power have turbines been known to utilize': 
380. How are steamboats moved ? What disadvantage do the paddles labor under? 
What is substituted in some vessels for the paddle-wheel ? What is a vessel moved 
by a screw call ?d '? 331. On what does the resistance a moving vessel encounters from 



BARKER'S MILL. 



161 



trates the water, on the principle of the wedge. Too great 
narrowness, on the other hand, is dangerous in boats that 
navigate stormy waters, and does not allow sufficient room 
for freight. To determine the shape that best combines 
speed, safety, and capacity, is the work of the ship-builder. 
It is a difficult problem, and one that is perhaps not yet 
solved, though great advances have been made of late years 
in naval architecture. 

382. Barker's Mill.- — An ingenious hydraulic machine, 
called Barker's Mill, and represented in Fig. 171, remains 
to be described. 

A is an upright hollow cylinder, turning freely 
on a vertical axis. Through its lower end runs a 
horizontal tube, B C, communicating internally with 
the cylinder. On opposite sides of this tube, at its 
extremities, are two small openings. A continuous 
stream is introduced, through the pipe D E, into the 
funnel at the top of the cylinder A. It runs down 
into the cross-tube B C ; and, if there were no op- 
portunity of escape, it would there rest, pressing 
equally in every direction. The moment, however, 
that the two holes in the ends are opened, the wa- 
ter runs through ; and the pressure at the holes be- 
ing thus removed, while that on the opposite sides 
remains undiminished, the tube is forced round in 
the direction of the pressure, that is, in an opposite 
direction to the jets of water. The cylinder A turns 
with the tube, and thus motion is communicated to 
the mill-stone S. H is a hopper, which feeds the 
mill with grain. 

383. Machines for Raising Water. barker's mill. 
— It is often desirable to raise water from a lower to a 
higher level. Well-sweeps, acting on the principle of the 
lever, are used for this purpose, as is also the wheel and 
axle in a variety of forms. But, when a large supply is re- 
quired, other machines, worked with less expense of time 
and labor, are employed. Some of these involve the prin- 
ciples of Pneumatics, and will be treated under that head. 




the water depend? What is the advantage, and what the disadvantage, of narrow- 
ness and sharpness of prow? 382. Describe Barker's Mill, and its mode of operation. 
883. What machines are used for raising water ? 884. What is one of the simplest 
11 



162 



HYDRAULICS. 



Those that belong exclusively to Hydraulics are described 
below. 

384. Archimedes' Screw. — The Screw of Archimedes, 
called after the philosopher that invented it, is one of the 
simplest machines for raising water. It consists of a tube 
wound spirally round a solid cylinder, as represented in 
Fig. 172. 

Fig. 172. /^ To work the machine, le 

one end of the tube, C, rest 
just below tbe surface of the 
water. The cylinder, A B, 
must be inclined at an angle 
of about 35 degrees, and be 
fastened at the lower end in 
such a way as to revolve 
freely when turned by the 
handle, H. When the cylin- 
der is turned, the open end 
of the tube, C, scoops up 
some of the water. When 
it has got half way round, 
the point D is lower than the 
and C, and the water descends to D by the force of gravity. Another half- 
revolution brings the point E lower than D, and again the water descends. 
This is continued till the water is discharged at the upper end. As new wa- 
ter is constantly scooped up, there will be a continuous flow as long as the 
handle is turned. — Archimedes' Screw operates only at short distances. 

385. The Chain Pump. — The Chain Pump is much 
used for raising water. The principle it involves is also 
applied in dredging-machines, for cleaning out the channels 
of rivers. 

This machine (see Fig. 173) consists of a continuous chain, to which cir- 
cular plates, c, d, e,f, &c, are attached at equal distances. The plates are 
of such a size as exactly to fit the cylinder G H, the lower end of which rests 
in the water. The chain passes over the two wheels, I, J ; to the upper one 
of which, I, a handle is attached. When the handle is turned, the chain is 
set in motion. The plates, ascending through G H, carry up water befor* 
them, which has no opportunity of escaping till it reaches the opening K. 




ARCHIMEDES' SCKEW. 



machines for raising water? Of what does Archimedes'' Screw consist ? Describe its 
mode of operation. At what distances does Archimedes' screw operate ? 385. What 
machine is much used for raising water ? What other application is made of the 
principle it involves? Describe the Chain Pump, and its mode of operating. 



THE HYDRAULIC RAM. 



163 



There it is discharged, as long as the Fig. 173. 

handle is turned. 

386. The Hydraulic Mam. 
— The Hydraulic Ram was in- 
vented in France, in 1796. It 
raises water by successive im- 
pulses, which have been com- 
pared to the butting of a ram, 
and hence its name. The re- 
quisite power is gained by mo- 
mentarily stopping a stream in 
its course, and causing its mo- 
mentum to act in an upward 
direction. 

Fig. 174 represents a simple form of 
the Hydraulic Ram. To a stream or res- 
ervoir at A, is adapted an inclined pipe, 
B, through which the water that works 
the ram is conveyed. Near the lower 
eud of the pipe B rises an air-chamber, 
D, with which an upright pipe, F, is con- 
nected. The passage connecting B with 
the air-chamber is commanded by a valve 
opening upward. At the extremity of 
the pipe B is another valve, E, opening 
downward, and made just heavy enough 
to fall when the water in B is at rest. 

Fig. 174. The play of the valve E makes the machine self- 

n F acting. Suppose the pipe B to be filled from the res- 

^ aas ^ ervoir ; the valve E opens by its weight, and allows 

jf ^ some of the water to escape. Soon, however, the 

water acquires momen- 
tum enough to raise the 
valve and close the open- 
ing. The stream is thus 
suddenly stopped, and 
wtpwb the pipe would be in 
danger of bursting from 
the shock were it not for 
the valve in the air-chamber D, which is at once forced upward, and allows 




THE CHAIN 




THE HYDRAITLIO RAM. 



386. When and where was the Hydraulic Ram invented ? Why is it so called ? 
How is the requisite power gained in the ram? Describe the hydraulic ram, and 



164 HYDRAULICS. 

some of the water to enter. The air in D is at first condensed by the pressure 
cf the water thus admitted ; but, immediately expanding by reason of its elas- 
ticity, it drives the water into F, for the closing of the valve prevents it from 
returning to B. By this time the water in B is again at rest, the valve E 
opens, and the whole process is repeated. 

By successive impulses the water may be raised in F to a great height. A 
descent of four or five feet from the reservoir is sufficient. Care must be 
taken to have the valve E just heavy enough to fall when B is at rest, and 
not so heavy as to prevent it from readily rising as the momentum of the 
stream increases. The pipe B must also be of such length that the water, 
when arrested in its course, may not be thrown back on the reservoir. 

387. Hydraulic Rams afford a cheap and convenient 
means of raising water in small quantities to great heights, 
wherever there is a spring or brook having a slight eleva- 
tion. They are used for a variety of purposes, and partic- 
ularly when a supply of water is needed for agricultural 
operations. 

EXAMPLES FOR PRACTICE. 

§3F" Friction is left out of account in these examples. 

1. {See §356, rule in italics.) Two streams issue from different orifices in the 

same vessel with velocities that are to each other as 1 to 6. How many 
times farther from the surface is the one than the other ? 

2. The stream A runs from an orifice in a vessel three times as fast as the 

stream B. How do their distances below the surface of the liquid com- 
pare ? 

3. In a vat full of beer there are two orifices of equal size; one 9 inches be- 

low the surface, and the other 25. How does the velocity of the latter 
compare with that of the former? 

4. There are three apertures in a reservoir of water, 1, 4, and 16 feet below 

the surface. With what comparative velocity will their streams flow ? 

5. A stream flows from an aperture in a vessel at the rate of 4 feet in a sec- 

ond. I wish to have another stream from the same vessel with a velo- 
city of 16 feet per second. How much farther below the surface than the 
first must it be? 

6. (See § 359.) A vat full of ale, 3 feet high, has four apertures in it, 3, 12, 18, 

and 24 inches respectively from the top. Through which will the liquid 
spout to the greatest horizontal distance ? Which next? Which next? 
% (See § 360.) How much water will be discharged every minute from an 
orifice of 3 square inches, the stream flowing at the rate of 5 feet in a 
second, and the vessel being kept replenished ? 



its mode of operating. How great a descent is required ? What precautions are 
necessary? 387. In what case may hydraulic rams he used with advantage ? 



EXAMPLES EOE PRACTICE. 165 

How much will be discharged every minute from another orifice in 
the same vessel, equally large, but situated four times as far below the 
surface of the liquid ? 

A stream flows from a hole in the bottom of a vessel with a velocity of '5 
feet in a second. The hole has an area of 5 square inches, and the ves- 
sel is emptied in 15 seconds. How much water does the vessel hold ? 

f 8ee § 376.) A stream having a momentum equivalent to 100 units of work 
is applied to an Undershot Wheel ; how many units of work will it per* 
form? — Arts. 25. 

See % 377.) How many units of work will it perform, if applied to an Oveft 
shot Wheel ? 

{See § 378.) How many, if applied to a Breast-wheel? 

(See §379.) How many, if applied to a Turbine? 



CHAPTER XII. 

PNEUMATICS. 

388. Pneumatics is the science that treats of air and the 
other elastic fluids, their properties, and the machines in 
which they are applied. 

389. Division of Elastic Fluids. — The elastic fluids 
are divided into two classes : — 

I. Gases, or such as retain their elastic form under ordi- 
nary circumstances. Some of the gases, under a 
high degree of pressure, assume a liquid form ; as, 
carbonic acid and chlorine ; others, such as oxy- 
gen and nitrogen, can not be converted into liquids 
by any known process. 
II. Vapors, or elastic fluids produced by heat from 
liquids and solids. When cooled down, they re- 
sume the liquid or solid form. Steam, the vapor 
of water, is an example. 

390. All gases and vapors have the same properties. 

8S8. What is Pneumatics ? 389. Into what two classes are elastic fluids divided f 
What are gases ? What difference is there in the gases ? What are vapors ? 390. In 



166 



PNEUMATICS. 



The principles of Pneumatics, therefore, relate to all alike, 
though they are most frequently exhibited and applied in 
the case of air, with which we have far more to do than 
with any other elastic fluid. 

Air. 

391. Air is the elastic fluid that we breathe. It sur- 
rounds the earth to a distance of about fifty miles from its 
surface, and forms what is called the Atmosphere. It exists 
in every substance, entering the minutest pores. 

392. Vacuums. — Air may be removed from a vessel with 
an instrument called the Air-pump. A Vacuum is then said 
to be produced. Vacuums sometimes result from natural 
causes ; but they last only for an instant, as the surround- 
ing air at once rushes in to fill them. Hence the old phi- 
losophers used to say, Nature abhors a vacuum. 

393. Pkoperties of Air. — Air can not be seen, but it 
can be felt by moving the hand 
rapidly through it. It is there- 
fore material, and has all the 
essential properties of matter. 

394. Air is impenetrable. 
595. T7i4 Diving-bell. — The impen- 
etrability of air is shown by the Diving- 
bell, represented in Fig. 175. A C is a 
large iron vessel, shaped somewhat like 
an inverted tumbler, and attached to a 
chain, by which it is let down in the 
water. As the vessel descends, the air 
in it is condensed by the upward pres- 
sure of the liquid, and water enters. 
The lower it gets, the more the air 
is compressed, and the greater the 
amount of water admitted. The im 
penetrability of the air, however, 
keeps the greater part of the bell 



Fig. 175. 




THE DIVING-BELL. 



what are the principles of Pneumatics most frequently exhihited, and why': 
891. What is Air? How far does it extend from the earth's surface? What does it 
constitute? 392. What is a Vacuum? What did the old philosophers say, and wh/? 
893. What proves the air to be material ? S94. What apparatus shows the impenetra- 
bility of air' 395. Describe the Diving-bell. Explain how descents are made with 



PROPERTIES OF AIR. 



167 



clear of water, so that several persons may descend in it to the bottom of 
the sea. 

As fast as the air is vitiated by the breath, it is let off by a stop-cock, 
while fresh air is supplied from above by a condensing syringe, through the 
pipe B. Air may be thus forced down in sufficient quantities to expel the 
water altogether from the bell, so that the divers can move about without 
difficulty on the bottom of the sea. If air were not impenetrable, the be'.l 
would be filled with water, and the divers drowned. 

When the diving-bell was invented, is not known. History makes no 
mention of it before the sixteenth century. At that time, we are told, two 
Greeks, in the presence of the emperor Charles V. and several thousand spec- 
tators, let themselves down under water, at Toledo in Spain, in a large in- 
verted kettle, and rose again without being wet. In 1665, a kind of bell was 
employed off the Hebrides, for the purpose of recovering the treasure lost 
in several ships belonging to the Invincible Armada. From that time to the 
present, various improvements have been made in the diving-bell ; and it is 
now extensively used for clearing out harbors, laying the foundation of sub- 
marine walls, and recovering articles lost by shipwreck. 

396. Air is compressible. 

This is proved with the diving-bell. If the air 
were not compressible, no water would enter the 
bell as it descended. 

397. Air is elastic. 

This also may be shown with the diving- 
bell. When, on its descent, water has entered, 
on account of the air's being compressed, let the 
bell be raised, and the air will resume its origi- 
nal bulk, expelling the water. 

Bottle Imps. — The compressibility and elasticity of air may 
be exhibited in an amusing way with the apparatus represent- 
ed in Fig. 176. In a vessel nearly full of water are placed sev- 
eral small balloons, or hollow figures of men, &c, made of col- 
ored glass, and called Bottle Imps. Each figure has a little 
hole in the bottom, and is of such specific gravity that it will 
just float in water. A piece of thin india rubber is tied over 
the mouth of the vessel, so as to cut off communication with 
the external air. Now press on the india rubber cover. The 
water at once transmits the pressure to the air in the hollow figures. This 
air is condensed, water enters, the specific gravity of the figures is increased, 






BOTTLE IMPS. 



It. What is the first mention made of the diving-hell in history? In 1665, for what 
purpose was it used ? For what is it now extensively used ? 396. How does the 
diving-bell prove air to be compressible ? 397. How does it prove air to be elastic? 
What properties in air do the Bottle Imps illustrate ? Describe the bottle imps, and 



168 PNEUMATICS. 

and they descend. On removing the fingers from the cover, the air, by rea- 
son of its elasticity, resumes its original bulk, and the figures rise. By thus 
playing on the india rubber, the figures may be made to dance up and down. 

398. Marlotte's Law. — The elastic fluids are the most 
easily compressed of all substances. The greater the pres- 
sure to which they are subjected, the less space they occupy, 
and the greater their density. A body of air which under 
a certain pressure occupies a cubic foot, under twice that 
pressure will be condensed into half a cubic foot ; under 
three times that pressure, into one-third of a cubic foot, &c. 
This principle, variously stated, is called, from its discov- 
erer, Mariotte's Law. 

The more the elastic fluids are compressed, the greater 
is their resistance to the pressure. Hence, their elastic force 
increases with their density. 

399. The Air-gun. — By subjecting a body of air to a great pressure, we 
may increase its elastic force sufficiently to produce wonderful effects. The 
Air-gun is an example. It consists of a strong metallic vessel, into which 
air is forced till it is in a state of high condensation. The vessel is then at- 
tached to a barrel like that of an ordinary gun, to the bottom of which a bul- 
let is fitted. Pulling a trigger opens a valve, the condensed air rushes forth, 
and drives the bullet »ut with great force. 

One supply of condensed air is sufficient for several discharges, though 
each is weaker than the preceding one. The labor required for condensing 
the air prevents this instrument from being much used ; but as it makes less 
noise, when discharged, than the ordinary gun, it is sometimes employed by 
assassins. 

400. Air has weight. 

Weigh a flask full of air, and then weigh the same flask 
with the air exhausted. The difference indicates the weight 
of the air contained. 

401. Experiments show the weight of 100 cubic inches of air to be about 
•31 grains. This makes it 815 times lighter than water. It has been com- 
puted that the weight of the whole atmosphere surrounding the earth is equal 
to that of a globe of lead 60 miles in diameter. 



explain the principle on which they dance up and down. 398. What substances are 
the most easily compressed ? What is Mariotte'a Law ? To what is the elastic force 
of gases and vapors proportioned ? 399. How may a body of air be made to produce 
wonderful effects ? What instrument proves this ? Describe the Air-gun, and its 
operation. Why is not the air-gun used more ? By whom is it sometimes employed ? 
W0. Prove that air has weight. 401. What is the weight of 100 cubic inches of air? 



ATMOSPHERIC PRESSURE. 



169 



Atmospheric Pressure. 

402. The particles of air, like those of the other elastic 
fluids, mutually repel each other. The atmosphere would 
therefore spread out into space, and become exceedingly 
rare, if it were not for the attraction of the earth. This 
prevents it from extending more than fifty miles from the 
surface, and gives it weight. 

403. Since air has weight, it exerts a pressure on all 
terrestrial bodies. This is known as Atmospheric Pressure. 
The pressure on any given body is equal to the weight of 
the column of air resting upon it, and therefore Fig. in. 
varies according to its size. 

404. Experiments. — The pressure of the 
atmosphere is proved by experiments. 

Experiment 1. — Take a common syringe, represented in 
Fig. 177, and let the piston, P, rest on the bottom of the bar- 
rel. Insert the nozzle, 0, in a vessel of water, and raise the 
piston. The water enters through 0, and follows the piston, 
as shown in the Figure. 

"What causes the water to rise? The piston, being air- 
tight, as it is drawn up, leaves a vacuum behind it ; and the 
pressure of the atmosphere on the water in the vessel drives 
it into the barrel through 0. If the piston does not fit the 
barrel tightly enough to exclude the air above, no water 
enters, because the pressure of the air from without is then 
counterbalanced by that from within the barrel. 

Exp. 2. — Take a small tube, close one end with the 
finger, fill it with water, and carefully invert it, as 
shown in Fig. 178. The water is kept in the tube by 
atmospheric pressure. Remove the finger, and the 
downward pressure of the atmosphere, which was be- 
fore cut off, will counterbalance the upward pressure, 
and the water will fall by its own weight. 

Exp. 3.— Fill a wine-glass with water, and cover the 
nouth with a piece of stiff paper. Place the hand over 
tlie paper, and invert the glass. On carefully removing 




What is the weight of the whole atmosphere? 402. What prevents the atmosphere 
from spreading out into space ? 403. What is Atmospheric Pressure ? What causes 
atmospheric pressure ? To what is the atmospheric pressure on any given body 
equal ? 404 Describe the experiment with the syringe that proves the pressure of 
tfie atmosphere. What will prevent the water from rising in the syringe ? Describe 

8 



170 



PNEUMATICS. 




the hand, the water will be found to remain in the glass, supported there by 
atmospheric pressure. 

Fig. 179. -^ X P- *• — When we raise the top board, 

A, of a common bellows (see Fig. 179), the 
valve B in the lower board opens. This is 
because a vacuum is formed within the bel- 
lows, and the atmospheric pressure forces 
the bellows. the valve up and drives in a portion of the 

external air. 
The same principle is involved in the act of breathing. The cells in the 
iungs are expanded by muscular action, a vacuum is thus formed, and the 
pressure of the atmosphere drives in the outer air through the nose or mouth. 
In a few seconds the muscles contract, and the same air, laden with impuri- 
ties received from the blood in the lungs, is expelled.' 



Fig. 180. 




THE 8TTCKEB. 



405. The Sucker, a play-thing used by- 
boys, shows the force of atmospheric pres- 
sure. It consists of a circular piece of 
leather with a string attached to the mid- 
dle. The leather, being first wet so that it 
may adapt itself to the surface, is pressed 
firmly upon a flat stone. The string is then 
gently pulled, so as to form a vacuum be- 
tween the leather and the stone. On this, 
the atmospheric pressure from above, not 
being counterbalanced from beneath, acts 
on the leather with such force that a stone 
of great weight may be lifted without the sucker's becom- 
ing detached. If a hole is made in the leather, air rushes 
in, the pressure from above is counterbalanced, and the 
stone falls by its own weight. 

It is by means of atmospheric pressure that the shell-fish called limpets 
fasten themselves so firmly to rocks, their feet acting like suckers, and 
vacuums being formed beneath when an attempt is made to remove them. 

406. Supported by the pressure of the atmosphere below, while it is cut 
off from that above, a liquid will not flow from the tap of a barrel unless a 
small opening is made in the top. As soon as this is done, air is admitted, 

the experiment with a small tube that proves the pressure of the atmosphere. Hovr 
may water be supported in a wine-glass by atmospheric pressure ? How is the pres- 
sure of the atmosphere exhibited with a common bellows ? How do we breathe ? 
405. Explain the principle involved in the Sucker. How do limpets fasten themselves 
to rocks ? 406. Why, when a barrel is tapped, must a hole be made in the top ? 



THE BAROMETER. 



171 



Fig. 181. 



the upward pressure is counterbalanced, and the liquid flows continuously. 
On the same principle, a small hole is made in the lid of a tea-pot. 

407. The Barometer. — The pressure of the atmosphere 
differs at different times and different places. To measure 
it, an instrument called the Barometer is used. 

The barometer was invented about the middle of the 
seventeenth century. It was the result of a celebrated ex- 
periment performed by Torricelli \to-re-chel' -le\, the friend 
and pupil of Galileo. 

408. Torricellian Experiment. — The Duke of Tuscany, having dug a well 
of great depth, and tried to raise water from it with an ordinary pump, found 
to his surprise that the water would not rise more than 32 feet. Galileo, to 
whom the fact was referred, was unable to explain it ; but shortly before his 
death he requested Torricelli to investigate the subject. Torricelli, 
suspecting that the water was raised and supported by atmospheric 
pressure, proceeded to test the truth of his opinion by experiment- 
ing with a column of mercury. Mercury is nearly 14 times as heavy 
as water; if, therefore, atmospheric pressure supported a column 
of water 32 feet high, it would support a column of mercury only 
about one-fourteenth of that height, or 28 inches. Accordingly, he 
procured a tube 3 feet long, sealed at one end ; and having filled it 
with mercury, and stopped the open end with his finger, he invert- 
ed the tube in a vessel of mercury, as shown in Fig. 181. When he 
removed his finger, the mercury fell, and finally settled, as he had 
supposed it would, at a height of about 28 inches, leaving a vacuum 
in the upper part of the tube. This is the famous Torricellian Vac- 
uum. 

Torricelli did not live to follow up his discovery ; but the French 
philosopher, Pascal, succeeded him with a variety of ingenious ex- 
periments. It occurred to Pascal that, if the columns of water and 
mercury were supported by the pressure of the atmosphere, then 
at great elevations, where this pressure would necessarily be less, 
the height of the columns supported would also be less. He tried 
the experiment on a mountain in Auvergne [o-varn']. At the foot 
of the mountain, the mercury stood at 28 inches ; at the top, it was 
below 25 ; and at intervening distances it stood between the two. 
This proved beyond doubt that the atmosphere exerted a pressure, 
and that this pressure varied according to the distance above the 
ievel of the sea. Perceiving how valuable such an instrument would be for 



407. What is the Barometer? When was it invented? Of what was it the result? 

408. Relate the circumstances that first directed attention to the subject. Give an 
account of Torricelli's experiment. What is meant by the Torricellian Vacuum ? 
Who followed up TorricehTs discovery? Give an account of Pascal's experiment 



172 



PNEUMATICS. 



measuring heights, Pascal soon constructed a Barometer, consisting of a tube 
and vessel of mercury so attached as to be conveniently carried. 

409. Kinds of Barometers. — There are several kinds 
of barometers. The simplest consists of Torricelli's tube 
and vessel of mercury, with a graduated scale attached to 
the upper part. The mercury never rises above 31 inches, 
and seldom falls below 27. The scale is therefore applied 
only to that part of the tube which lies 
between these limits. 

The Wheel Barometer is exhibited 
in Fig. 182. 

Here the tube, instead of resting in a vessel of 
mercury, is bent upward at its lower extremity. 
A float, F, is supported by the mercury in the short 
arm of the tube. To this float is attached a thread, 
which passes over the pulley P, and is attached to 
the ball W. When the mercury falls in the long 
arm of the tube, it must rise in the short arm, and 
with it rises the float F. The thread turns the 
pulley P, and this moves the index I, which is so 
arranged as to traverse the graduated scale S S. 

410. The Barometer as a Weather- 
guide. — The barometer shows that the 
pressure of the atmosphere at any 
given place is different at different 
times. This is because the air is con- 
stantly varying in density, on account 
of a greater or less intermixture of for- 
eign substances. When the air is 
densest, the mercury stands highest, 
and we generally have clear weather ; 
but, when the air is rarefied, the mer- 
the wheel barometek. cury falls, and rain not unfrequently 
follows. Hence, the barometer has been used for predict- 




WTiat did it prove ? 409. Of what does the simplest kind of barometer consist ? To 
what part of the tube is the scale confined, and why ? Describe the Wheel Barom- 
eter, and its mode of operation. 410. What does the barometer show with respect to 
the pressure of the atmosphere ? What occasions this difference ? When the air is 
densest, what generally follows ? When it is rarefied, what follows ? In view of this, 



THE BAROMETER. 173 

ing changes of weather; and the words fair, change, rain, 
are placed at different points on the scale, to indicate the 
weather which may be expected when the mercury reaches 
either of those levels. 

411. The only reliable indications, however, afforded by the barometer 
are changes in the level of the mercury. No regard should be paid to the 
particular point at which it stands at any given time ; we should merely ask, 
is it rising or falling ? The following rules generally hold good : — 

1. After much dry weather, if the mercury falls steadily, rain will ensue, 

though it may not begin for several days. The longer it is in com- 
ing, the longer it will last. 

2. After much wet weather, if the mercury, standing below its medium 

height, rises steadily, fine weather will ensue, though it may not be- 
gin for several days. The longer it is in coming, the longer it will last. 

3. A sudden fall of the barometer, in spring or fall, indicates wind ; in 

very hot weather, a thunder-storm ; in winter, a change of wind, and 
rain or snow according to the temperature. 

4. Sudden changes of the mercury indicate violent changes of the weather, 

but not permanent ones. 

5. A rise of mercury in autumn, after much wet and windy weather, indi- 

cates the approach of cold. 

412. At sea, the barometer may be relied on with tole- 
rable certainty, and it is therefore exceedingly useful to 
navigators. Violent and frequent changes in the mercury 
almost invariably precede a sudden storm. Warned in 
time, the prudent mariner furls his sails, and thus escapes 
the fury of the hurricane which would have proved fatal to 
his craft had it struck her unprepared. 

Dr. Arnott gives the following account of his preservation at sea through 
the warning of the barometer : — " It was in a southern latitude ; the sun 
had just set with placid appearance, closing a beautiful afternoon ; and the 
usual mirth of the evening watch was proceeding, when the captain's order 
came to prepare with all haste for a storm : the barometer had begun to fall 
with appalling rapidity. As yet the oldest sailors had not perceived even a 
threatening in the sky, and were surprised at the extent and hurry of the 
preparation ; but the required preparations were not completed, when a more 
awful hurricane burst upon them than the most experienced had ever braved. 

to what use has the barometer been applied ? 411. What are the only reliable indi- 
cations afforded by the barometer ? What does a steady fall of mercury in the ba- 
rometer after much dry weather indicate ? What does a rise of mercury after much 
wet weather indicate ? What does a sudden fall indicate at the different seasons? 
What do sudden changes indicate ? What does a rise of mercury in autumn indicate ? 
412. What is 6aid of the barometer at sea? Relate the circumstances of Dr. Arnott's 



174 



PNEUMATICS. 



Fie. 183. 



Nothing could withstand it ; the sails, already furled and closely bound to 
the yards, were riven away in tatters ; even the bare yards and masts were 
in great part disabled, and at one time the whole rigging had nearly fallen 
by the board. In that awful night, but for the little tube of mercury which 
had given the warning, neither the strength of the noble ship nor the skill 
and energies of the commander could have saved one man to tell the tale." 

413. Density of the Air at different Levels. — The 
ovvest parts of the atmosphere are the densest, as they 

have the greatest quanti- 
ty of air pressing on them 
from above. 

414. At the level of 
the sea, the pressure of 
the atmosphere on every 
square inch of surface is 
15 pounds. The body of 
a man of ordinary size has 
a surface of about 2,000 
square inches, and is there- 
fore subjected to the enor- 
mous pressure of 30,000 
pounds. We do not feel 
this pressure, because it is 
counterbalanced by that 
of the air within our 
bodies. 

415. The higher we go 
above the level of the sea, 
the less is the pressure of 
the atmosphere and the 
rarer the air. At an ele- 

|| vation of 18 miles, the 
mercury would fall to 1 
inch, — that is, the air 
above that point is so rare, 




1. Highest Peak of the Himalayas. 

2. Highest Peak of the Alps. 
Q. Highest Peak of the Andes. 

4. Mount Mitchell, N. Carolina. 



preservation at sea by means of the barometer. 413. What parts of the atmosphere 
are densest, and why ? 414. How great is the pressure of the atmosphere at the level 
of the sea? How great is the pressure on the body of a man of ordinary size ? Why 



DENSITY OF AIE AT DIFFERENT LEVELS. 175 

that a column of it 30 miles high weighs no more than an 
equal column of mercury 1 inch in height. 

The shading in Fig. 183 shows the gradual increase in the density of 
the air as the surface of the earth is approached. The figures in the left 
margin represent the height of the atmosphere in miles ; those on the 
right, the corresponding height, in inches, at which the mercury stands in 
the barometer. On the top of Mount Mitchell, the most elevated peak in 
the United States east of the Mississippi, somewhat over a mile and a 
quarter high, it stands at 24 inches ; on the highest peaks of the Himalayas, 
which are more than five miles high, at no more than 12. 

416. The rarity of the air is painfully felt by those who 
ascend to great heights on mountains. The pressure of the 
external air being diminished, that which is in the body 
expands, the delicate blood-vessels burst, the skin cracks, 
and blood issues from the nose and ears. Among the Andes, 
the Indians are subject to a malady called veta, which is 
caused by the rarity of the air. The head aches violently, 
its veins are swollen, the extremities grow cold, and breath- 
ing becomes difficult. 

Effect of Heat on Air. 

417. Air is rarefied by heat. 

Throw some burning paper into a wine-glass, and before the flame goes 
out place your hand over the top. The glass will be found to adhere to your 
hand. This is because the heat rarefies the air within, and thus expels most 
of it before the top is covered. The pressure of the external air, not being 
counterbalanced by any pressure from within, fastens the glass and hand 
together. 

418. Cupping-glasses are made to draw on this principle. Incisions hav- 
ing been made in the skin, the sides of the glass are moistened with alcohol, 
and flame is applied. While the alcohol is burning, the glass is inverted on 
the skin. The pressure of the air in the body, no longer counterbalanced by 
the external pressure, causes a flow of blood into the cup. 

419. Heated air, being lighter than that which surrounds 



lo we not feel this pressure ? 415. What is said of the air, as we ascend above the 
sea-level ? How would the mercury stand at a height of 18 miles ? What does Fig. 
188 show ? How does the mercury in the barometer stand on the top of Mount 
Mitchell? On the tops of the Himalayas? 416. What sensations are experienced 
by persons who ascend to great heights on mountains ? Describe the symptoms of 
the veta. 417. What is the effect of heat on air ? How may the rarefaction of air 
by heat be shown ? 418. Explain the operation of cupping-glasses. 419. Why does 



176 



PNEUMATICS. 



Fig. 184. 




it, ascends till it reaches a region of the atmosphere as rare 
as itself. 

This is the reason why smoke rises. So, when a fire is kindled in a grate, 
a draft is produced in the chimney. The air near the fire is rarefied and as- 
cends. A vacuum is thus formed for the instant ; cold air rushes in to fill it ; 
this in turn is heated and rises, and thus there is a constant passage of hot 
air up through the chimney. 

To show the ascent of hot air, take a circular 
piece of paper, as represented in Fig. 184, and, 
commencing at any point of the outer edge, as 
A, cut in the direction of the dotted line. Sup- 
port it from beneath at B on a piece of wire, and 
it will hang down, resembling in shape the 
threads of a cork-screw. If the paper thus sus- 
pended be held over a hot stove, it will be carried 
rapidly round by the ascending currents of heat- 
ed air. 

420. Balloons. — By observing the rise of smoke, Ste- 
phen and Joseph Montgolfier \mon-gol-fe-a!\ paper-manu- 
facturers in France, were led in 1*782 to the invention of 
balloons. The following year, they exhibited their invention 
to the public. 

An immense bag of linen lined with paper was prepared, and brought di- 
rectly over a fire of chopped straw. In a few minutes, the balloon was filled 
with rarefied air and released from its fastenings. It rose about a mile, re- 
mained suspended ten minutes, and reached the ground a mile and a half 
from the place of its ascent. The same year, two persons ascended to a 
height of 3,000 feet in the basket of a smoke balloon, and came down in safety. 

On the 1st of January, 1784, a successful ascent was 
made in a balloon inflated with hydrogen. This gas is now 
generally used for the purpose, on account of its superior 
buoyancy. Even when badly prepared, it has but one-sixth 
of the weight of air, and is three times as light as Montgol- 
fier's mixture of heated air and smoke. 

421. Balloons have not as yet been turned to any practical use, from the 
fact that they are completely at the mercy of the wind, no way of steering 
them having been devised. A theory has lately been put forth, however, 
that at a certain height of the atmosphere currents are always setting from 



heated air rise ? Explain how the kindling of a fire causes a draft in a chimney. How 
may the ascent of hot air he shown ? 420. By whom and when were balloons invent- 
ed? Describe the Montgolfiers' balloon, and its ascent. When was the first success- 
ful ascent made in a balloon inflated with hydrogen ? Why is hydrogen now used for 



NAVIGATION OF THE AIR. 



177 



west to east ; if this "be so, aerial voyages may be made with, tolerable cer- 
tainty, at least in one direction. The theory in question was to some ex- 
tent confirmed by a balloon voyage (one of the most remarkable on record) 
made July 1, 1859. four persons started from St. Louis, and in 19 hours, 
40 minutes, landed in Jefferson Co., N. Y., near Lake Ontario, — having 
travelled about 1,000 miles, at a rate exceeding that of the fastest railroad 
train. 

422. Long before the invention of balloons, attempts were made tc 
navigate the air. At different periods not long after the Christian era, ad- 
venturous men launched themselves from the tops of high buildings, and 
with different sorts of apparatus which they had prepared moved a short 
distance through the air. Mechanical contrivances resembling wings were 
more than once resorted to ; but several who tried -them met with serious 
accidents, and it was at last proved that wings sufficiently large to support 
a man in the air would be too heavy for him to move. 



Fig. 185. 




A RECEIVER. 



The Air-pump. 

423. The Air-pump is an instrument 
used for removing the air from a vessel 
called a Receiver. Receivers are made 
of glass, and are usually of the shape 
represented in Fig. 185. 

424. Invention of the Aie-pump. — 
The air-pump was invented 1654 a. d., 
by Otto Guericke [c/d'-re-ka], burgomas- 
ter of Magdeburg, Germany. 

Guericke's first attempt to obtain a vacuum was made 
with a barrel full of water. Having closed it tightly, he 
applied a pump to the lower part and commenced drawing 
off the water. Could he have done this and kept the air 
out, a vacuum would have been formed ; but he had not 
proceeded far, when the air from without began to force 
its way with a loud noise through the seams of the barrel. 
To remedy the difficulty, Guericke substituted a metallic 
globe for his barrel of water, and the experiment was 
then successful. 



inflating balloons ? 421. Why have not balloons been turned to practical use ? What 
remarkable voyage was made in July, 1S59 ? 422. Give an account of the early at- 
tempts to navigate the air. 423. What is the Air pump ? Of what are receivers made ? 
424. By whom and when was the air-pump invented? Give an aocount of Guericke's 
first attempt to obtain a vacuum. How did he finally succeed? Describe Gue- 
12 



178 



PNEUMATICS. 




MAGDEBURG 
HEMISPHERES. 



Great improvements have been made on the rude air-pump employed by 

Guericke ; yet, imperfect as his instrument was, it produced results of deep 

Interest to the learned men of that day. His most famous experiment was 

performed before the Emperor of Germany and his court. Two hollow me- 

Fig. 186, tallic hemispheres of great size were prepared, fitting each 

other so closely as to form an air-tight globe. From this 

globe the air was removed with the pump, and a stop-cock 

prevented any new air from entering. Fifteen horses 

were then harnessed to each hemisphere ; but their united 

strength was unable to effect a separation, so tightly were 

the two parts held together by atmospheric pressure. On 

turning the stop-cock and readmitting the air, they fell 

asunder by their own weight. 

425. This experiment is often repeated at the present 
day, on a small scale. The Magdeburg hemispheres, as they 
are called from Guericke's native city, are represented in 
Fig. 186. They are fixed to the plate of an air-pump, in- 
stead of a receiver ; and on exhausting the air they are 
pressed together so tightly that two men can not pull them 
apart. 

426. Single-barrelled Air-pump. — A single-barrelled 
Fig. 187. air-pump is repre- 

sented in Fig. 187. 
A is a receiver with 
its edge carefully 
ground, resting on a 
plate near the centre 
of the stand. In 
this plate there is a 
hole leading into a 
pipe beneath, which 
connects the receiv- 
er with the barrel B. 
The lower part 
of the barrel is rep- 
resented as cut away 
in the figure, in or- 
der to show the interior. A piston is tightly fitted to it, 
containing a valve opening upward, and connected with a 




THE SINGLE-BARRELLED AIR-PTJMP. 



ricke's famous experiment before the Emperor of Germany. 425. Describe the ex- 
periment with the Magdeburg hemispheres. 426. Describe the single-barrelled air- 



THE AIR-PUMP. 



179 



handle, which is either like that shown in the figure, or more 
commonly a lever like a pump-handle. At the base of 
the barrel there is another valve, also opening upward. 

427. Operation.— The plate having been carefully dusted and rubbed with 
a little oil, the receiver is placed on it, and the piston is drawn up. A vac- 
uum is thus formed in the lower part of the cylinder, and the air in the re- 
ceiver, by reason of its elasticity, pushes up the lower valve and enters the 
barrel. The piston is now in turn driven down ; the pressure at once closes 
the lower valve, while the resistance of the air in the barrel opens the valve 
In the piston. Through the latter the air passes out, and by the time the 
piston has reached the bottom, it has all escaped. The piston is then again 
raised, and the whole operation is repeated, — a barrel-full of air being drawn 
out from the receiver as often as the piston ascends, and expelled from the 
barrel as it descends. At last the air in the receiver becomes so rare that it 
has not sufficient elasticity to open the valve at the base of the barrel. After 
this the exhaustion can not be carried any further. A perfect vacuum, there- 
fore, is not produced ; but the air is rarefied to such a degree that we speak 
of it as such. 

Fig. 188. 




THE DOUBLE-BARRELLED AIR-PUMP. 



180 PNEUMATICS. 

428. Double-barrelled Air-pump. — £The double-bar- 
relled air-pump (Fig. 188) acts on the same principle as 
the single-barrelled pump,, but exhausts the air more rapid- 
ly in consequence of having two barrels, A, B, and two 
pistons, C, D. Each piston is connected with a rack, E, F, 
the teeth of which work in a cog-wheel turned by the 
handle G M. When one piston is raised, the other is low- 
ered. H I is a passage which connects the barrels with 
the receiver J. K is a stop-cock, by which the connection 
may be cut off. Lisa barometer gauge, enclosed in a glass 
vessel which communicates with the receiver ; it is a bent 
tube, having one branch closed and filled with mercury, 
while the other is open. When the pressure of the air in 
the receiver becomes less than that of the column of mer- 
cury in the closed branch of the gauge, the mercury in 
the latter falls, and the elastic force of the air at any mo- 
ment is shown by the difference of level of the mercury. 

429. Experiments with the Air-pump. — With the air- 
pump and different pieces of apparatus which accompany 
it, may be performed a variety of experiments, illustrating 
the properties of air. 

Fic , 190 430. The Hand-glass— The Hand-glass (Fig. 190) 

is a receiver open at both ends. Set the large end 
on the plate of the air-pump, and place the hand 
flat upon the top. As soon as the pump is worked, 
the pressure of the atmosphere is felt. When the 
air is exhausted, the hand can hardly be removed 
from the glass ; on readmitting the air through a 
stop-cock, it is raised without difficulty. The ex- 
pansion of the air in the palm of the hand is shown 
the hand-glass ^y tne recm ess of the flesh, and its puffing out while 

over the exhausted glass. 
431. The Apple-cutter.— The Apple-cutter (Fig. 191) is a metallic cylindet 
with a sharp upper edge. An apple that fits it closely having been placed 
on its top, the air is exhausted. The pressure of the atmosphere forces the 
apple down on the sharp edge ; the middle part is cut out and falls inside of 
the vessel. 




pump, as shown in Fig. 187, and its mode of operation. Describe toe double-bar- 
relled air-pump, with the aid of Fig. 1S8. What is the use or' the barometer gauge ? 
430. What is the Hand-glass ? Describe the experiment with the hand-glass. What 
causes the redness of the hand ? 431. What is the Apple-cutter ? Describe the ex- 



EXPERIMENTS WITH THE AIR-PUMP. 



181 




Fig. 192. 




THE APPLE- 
CUTTER. 



THE BLADDEB- 

GLASS. 



Fig. 193. 



Fig. 191. 432. The Bladder-glass.— Over the large 

end of the hand-glass tie a wet bladder, as 
shown in Fig. 192. When the bladder has 
become dry, place the open end on the plate, 
and exhaust the air from the glass. The 
pressure of the atmosphere, unsupported 
from within, soon bursts the bladder with a, 
loud noise. If a piece of thin india rubber 
be substituted for the bladder, it will be 
drawn in and distended, till it covers near- 
ly the whole inside of the glass. 
433. The Lungs-glass. — The Lungs-glass (Fig. 193) illus- 
trates the elasticity of air. It is a small glass globe with 
a metallic stopper. Through this stopper passes a tube, 
to the lower part of which a bladder is tied. The whole is 
placed under a receiver, and the air exhausted. The air 
in the bladder, communicating through the tube with the 
receiver, is gradually rarefied. The air around it in the 
glass, having no communication 
with the receiver, remains of the . 
same density. Owing to its pres- 
sure, the bladder becomes shrivelled 
when the receiver is exhausted ; 
but, on the re'admission of the air, it resumes its former 
dimensions. This movement, regularly repeated, re- 
sembles the action of the lungs in breathing, and hence 
the name given to the apparatus. 

434. Vacuum Fountain.- — Fig. 194 represents a 
tall glass receiver, terminating at the bottom in a me- 
tallic cap, through which a tube passes. This tube is 
furnished with a stop-cock, and a screw, by means of 
which it may be fastened to the plate of an air-pump. 
A jet communicating with the tube rises into the re- 
ceiver. Screw this appai atus to the plate of the pump, 
exhaust the air, and close the stop-cock. Then un- 
screw the whole, place the lower end of the tube in a 
vessel of water, and open the stop-cock. The pres- 
sure of the atmosphere will force the water up through 
the tube and jet into the vacuum, forming a beautiful 
miniature fountain. 

Another mode of producing a vacuum fountain is 
with the apparatus shown in Fig. 195. It consists of 





THE LUNGS-GLASS. 



VACUUM FOUNTAIN. 



periment with the apple-cutter. 432. How is the experiment with the bladder-glass 
performed ? 433. What does the Lungs-glass illustrate ? What does it consist of? De- 
scribe the experiment Why is the lungs-glass so called ? 434. What does Fig. 194 rep- 
reseat ? How is the vacuum fountain produced ? Describe another mode of producing 



182 



PNEUMATICS. 




Fig. 195. a glass vessel with an air-tight stopper, through which a 
tube extends almost to the bottom. The vessel,, nearly filled 
with water, is placed under a tall receiver, and the air ex- 
hausted. The elasticity of the air within the vessel, not be- 
ing counterbalanced by any pressure from without, forces the 
water through the tube in the form of a fountain. 

435. Bottle Imps.— The bottle imps, described in § 397, 

may be made to dance up and down in a jar of water in an 

exhausted receiver. These figures are hollow and contain 

air. When the receiver is exhausted, the pressure on the surface of the 

water being removed, the air in the figures expands and drives out some of 

the water. This diminishes their specific gravity, and causes them to 

Fig. 196. r i se - When the air is readmitted, the pressure is restored, 

the air in the figures is compressed, water enters, their 

specific gravity is increased, and they sink. 

436. The Mercury Shower. — On an open-mouthed re- 
ceiver, D, place the cup A, in the bottom of which is a plug 
of oak wood, B, projecting downward about two inches. 
Put some mercury in A, and set the saucer C beneath the 
oaken plug. Exhaust the air from D, and the mercury 
will soon be forced by atmospheric pressure through the 
pores of the oak, and fall into the saucer in a silvery 
shower. 

437. The Weight-lifter. — This is an apparatus with 
which the pressure of the atmosphere is made to lift a 

heavy weight (see Fig. 197). A is a cylinder attached to a frame, firmly sup- 
ported by three legs. On the bottom of the cylinder rests a closely fitting 
piston, to which the platform F is attached. A tube, B C, connects the in- 
terior of the cylinder with the plate E of the pump D. When the air is ex- 
hausted from A, the pressure of the atmosphere raises the piston, together 
with the platform and its contents, the whole length of the cylinder. Atmos- 
pheric pressure being 15 pounds to the square inch, the number of pounds 
that can be lifted by a given cylinder may be found by multiplying its area 
expressed in inches by 15. 

438. Atmospheric pressure has been turned to practical account for the 
transmission of mails, on the principle of the weight-lifter. A strong 
metallic tube, perfectly smooth on the inside, is laid between two places, 
and a piston is tightly fitted to it. Large air-pumps, worked by steam, 
are placed at the ends of the tube. The mail being attached to the piston 
at one end of the line, the air-pump at the other is set in motion. A par- 
tial vacuum is produced, and atmospheric pressure drives the piston through 
the tube with great velocity. Atmospheric railways have been constructed 
on the same principle, a train of cars outside of the tube being connected, 

a vacuum fountain. 435. How may bottle-imps be made to dance up and down in a 
jar of water ? Explain the principle. 436. How is the mercury shower produced ? 
437. What is the Weight-lifter ? Describe it, and its mode of operating. How many 
pounds will a given cylinder lift ? 438. How are mails transmitted, and cars propelled 




EXPERIMENTS WITH THE AIR-PUMP. 183 

Fig. 19T. 




by an ingenious arrangement, with the air-tight piston propelled iai the 
manner described above. 

439. Vacuum Bell. — This apparatus is intended 
to show that air is essential to the production of 
sound. A bell is so fixed under a receiver that it 
can be rung by pushing down a sliding-rod which 
passes through the top. When rung before the re- 
ceiver is exhausted, the bell is distinctly heard; 
but, when the air is withdrawn, it is almost inaudi- 
ble. If a perfect vacuum could be produced, it would 
not be heard at all. 

440. Freezing Apparatus. — Water may be frozen 
in a vacuum, with the apparatus shown in Fig. 199. 

Having placed the liquid 
in a shallow vessel over a 
basin containing strong 
sulphuric acid, set the 
whole under a receiver 
and exhaust the air. Un- 
der the diminished pres- 

on railways, by atmospheric pressure ? 439. What is the apparatus known as the 
vacuum bell intended to show ? Describe the experiment. 440. Describe the freez- 



Fig. 199. 





184 PNEUMATICS. 

sure, the water is rapidly converted into vapor, which is as rapidly absorbed 
by the acid. The continued evaporation cools the water to such a degree 
that it is finally covered with ice. 

441. Miacdlaneous J^xperimeats. — In a vacuum, boiling commences at a 
much lower temperature than in the air. This is shown by placing some 
hot water under a receiver and exhausting the air. The pressure of the at- 
mosphere being removed from its surface, the water soon boils ; but it comes 
to rest the moment that air is readmitted. For the same reason, water boils 
sat a lower temperature on the top of a mountain than at its base, as has often 
been observed by travellers. 

442. If beer is placed under a receiver and the air exhausted, it begins to 
foam. This is owing to the elasticity of the carbonic acid in the liquid, rush- 
ing out to rill the vacuum. If the air is readmitted, the beer resumes its 
usual appearance. 

443. A shrivelled apple in an exhausted receiver is puffed out to its full 
size by the expansion of the air within. 

444. If a vessel of water containing a piece of wood, a vegetable, or al- 
most any solid substance, is placed under a receiver, and the air is exhaust- 
ed, minute globules of air can be seen forming on the surface of the solid, 
and sometimes even bubbling up through the water. This proves the poros- 
ity of solids and the presence of air in their pores. 

445. A lighted candle in an exhausted receiver is extinguished, and the 
smoke falls because it is heavier than the rarefied air. If a mouse, rabbit, or 
other living creature, is placed under a receiver and the air is drawn off, it 
immediately shows signs of distress, and soon dies. 

446. These experiments show that air is everywhere 
present, and is essential to life and combustion. In a vac- 
uum, animals die, vegetation ceases, and sound can not be 
produced. 

The Condenser. 

447. The Condenser (Fig. 200) is an instrument used 
for forcing a large quantity of air into a given vessel. 

Like the single-barrelled air-pump, the condenser con- 
sists of a cylinder, A, with a valve at its base, V, and a pis- 
ton, P, which also contains a valve, tightly fitted to it 



lug apparatus, and the experiment with it 441. At what temperature does boiling 
Commence in a vacuum, compared with that at which it commences in the air? 
How is this shown ? What is said of the boiling of water on the top of a mountain 
442. What phenomenon is presented when beer is placed under a receiver and the ail 
exhausted.' 44 >. When a shrivelled apple is so placed? 444. How is the presence 
of air in the pores of solids proved with the air-pump? 445. How is it shown with 
the air-pump that air is necessary to combustion and animal life ? 447. What is the 



THE CONDENSER. 



185 




Fig. 201. 



Instead of opening upward, however, as in the Fig. 200. 
air-pump, these valves open downward. 

448. Operation. — The condenser having been 
screwed to any strong vessel in which it is desired 
to condense air, the handle is worked up and 
down. A vacuum being produced below the pis- 
ton, as it ascends, its valve is opened and air 
Tushes in ; while the valve in the cylinder is closed 
by the pressure of the air in the vessel. When 
the piston descends, its valve is closed by the 
pressure of the air in the cylinder, while the other 
valve opens and allows this air to be driven into the vessel. 
With every ascent of the piston, therefore, the cylinder is 
filled with air, and with every descent 
this cylinder-full of air is forced into 
the vessel. 

Air is condensed in the chamber of 
the air-gun (described in § 399) by the 
use of this instrument. 

449. Experiment. — An interesting experiment 
may be performed with the condenser and the ap- 
paratus represented in Fig. 201. A is a globe half 
full of water, with a tube, B, reaching nearly to the 
bottom, and extending upward through an air-tight 
cap till it terminates in a screw just above the stop- 
cock D. The condenser, having been screwed on, 
is worked till a large quantity of air is forced into 
A. The stop-cock is then closed, the condenser is 
unscrewed, and a jet-pipe, C, is put on in its place. 
The stop-cock is now opened, when the pressure of 
the condensed air, being greater than that of the 
atmosphere, forces the water in A up through the 
jet, making a beautiful fountain. — This experiment 
shows that the elasticity of air is increased by 
oondensing it. 




Pneumatic and Hydraulic Machines. 

450. The Siphon. — The Siphon, represented in Fi< 



Condenser? Describe it. 448. How does the condenser operate ? 449. Describe an 
experiment with the condenser and the apparatus represented in Fig. 201. 450. "What 



186 



PNEUMATICS. 




THE 8IPH0N. 



202, is a simple instrument for drawing off liquids from a 
higher to a lower level. It is nothing more than a bent 
tube, with one leg longer than the other. 

To use the siphon, fill it with some liquid and then invert 
it, stopping the long end with the finger, and setting the short 
one in the liquid to be drawn off. Remove the finger, and the 
liquid will commence flowing from the long end. The upward 
pressure of the atmosphere is counterbalanced by its down* 
ward pressure on the surface of the liquid to be drawn off, and 
the liquid in the tube will therefore flow in the direction of its 
greatest weight. As it flows, a vacuum is formed in the tube, 
and fresh liquid is constantly forced up into the short leg. 
The flow continues till the liquid falls below the extremity of 
the short leg. 

451. Some siphons, like that in the figure, have an addi- 
tional tube, open at the upper end and at the lower communi- 
cating with the long leg. This saves the trouble of turning 
the siphon, every time it is used, to fill it with liquid ; for, 
the long leg being stopped with the finger and the mouth ap- 
plied to this additional tube, the liquid may by suction readily be made to 
fill both legs. 

452. Tantalus's Cup. — Fig. 203 represents Tantalus's 
Cup, which is simply a goblet containing a siphon, the short 
Fig. 203. leg of which reaches nearly to the bottom, 
while its long leg passes through the bottom 
and extends below. The siphon is concealed 
by a figure, which seems to be trying to 
drink. Water is poured in ; but, the mo- 
ment it reaches the lips of the figure, it re- 
cedes, because just then it passes the turn 
of the siphon and begins to be discharged 
below. 

tantalus's cup. 453. The Lipting-pump.— The Lifting- 

pump was invented by Ctesibius \te-sib' -e-us\, who flourished 
at Alexandria, in Egypt, 250 b. c. Though the son of a 
barber and brought up to his father's calling, he attained 
distinction by his mechanical abilities. Several ingenious 

is the Siphon ? How is it used ? Explain the principle on which it works. 451. What 
improvement is attached to some siphons ? 452. Describe Tantalus's Cup, and the 
principle on which it works. 453. Who invented the Lifting-pump ? What is said 
of Ctesibius ? 454. Of what does the lifting-pump consist ? 455. Describe its mode 




THE LIFTING-PUMP. 



187 



contrivances for raising water are attributed to this philos- 
opher, besides the clepsydra already described. 

454. The common Lifting-pump is rep- Fig. 204 

resented in Fig. 204. It consists of a cyl- 
inder, B C, to which is fitted the air-tight 
piston G, containing a valve opening up- 
ward. A is called the suction-pipe ; it 
must be long enough to reach the water 
that is to be raised. In the top of the 
suction-pipe is the valve H, opening upward 
into the cylinder. E is a handle, by which 
the piston may be worked. F is a spout, 
from which the water is discharged. 

455. Operation. — To work the pump, raise the pis- 
ton. As it ascends, it leaves a vacuum behind it, and 
the water under the pressure of the atmosphere rushes 
up through A, opens H, and fills the cylinder B C. The 
piston, having reached the top, is now forced back. 
Its downward pressure at once closes the valve H, so 
that the water can not return into the suction-pipe ; but 
the valve in the piston opens, and through it the water 
rushes above the piston. When the piston has reached 
the bottom of the cylinder, it is again raised ; its valve 
being now closed by the downward pressure, the water 
is lifted by the piston into the reservoir D, whence it is 
discharged by the spout. Meanwhile, the second time 
the piston rises, a vacuum is formed below it as before, 
and the whole operation is repeated. 

456. Thus we see that water is raised in pumps by at- 
mospheric pressure. The air will support a column of wa- 
ter from 32 to 34 feet high. The valve H of a lifting- 
pump cannot be higher than this above the water. To 
overcome much greater heights, the forcing-pump is used. 

457. The Forcing-pump. — The Forcing-pump, after rais- 
ing a liquid through its suction-pipe, does not discharge it 
from a spout above, but by the pressure of the returning 
piston drives it through an opening in the side below. The 




THE LIFTING-PUMP. 



of operation. 456. By what agency is water raised in pumps ? How high a column 
will tttmospheric pressure support ? To raise water to a greater height, what must 



188 



PNEUMATICS - 



Fig. 205. 



liquid is thus forced, either directly or by means of the 
pressure of condensed air, to a greater height than it could 
otherwise attain. 

458. Fig. 205 represents one form of 
the forcing-pump. It has a cylinder, pis- 
ton, and suction-pipe, like the lifting-pump 
just described; but there is no valve in 
the piston. Near the bottom of the cylin- 
der enters the pipe M, which communi- 
cates with the air-chamber K, by the valve 
P, opening upward. The tube I, open at 
the bottom and terminating at the upper 
end in a jet, passes through the air-tight 
top of the chamber K, and extends nearly 
to its bottom. 

459. Operation. — To work the forcing- 
pump, raise the piston. A vacuum is 
formed ; and water, from the reservoir 
below, rushes through the suction-pipe, 
opens H, and fills the cylinder. The pis- 
ton is now pushed back, when H at once 
closes. The water in the cylinder is forced 
into M, raises P, and enters the chamber 
K. The water in K soon rises above the 
mouth of the tube I, and begins to con- 
dense the air in the upper part of the chamber. The higher 
the water rises in K, the more the air is condensed, and its 
elasticity increases in proportion. Its pressure, therefore, 
soon becomes greater than that of the atmosphere, and drives 
out the liquid through the jet. 

Some forcing-pumps have no air-chamber, but drive out 
the liquid by the direct pressure of the descending piston. In 
that case, the discharge is by successive impulses ; but, when 
made from an air-chamber, it is continuous. 

460. The Fire-engine. — The Fire-engine is a combina- 
tion of two forcing pumps, -with a common air-chamber 
and suction-pipe. Its operation will be understood from 
Fig. 206. 

The pistons, C, D, are attached to a working-beam, A B, turning on the 




THE FORCING' 
PUMP. 



be used? 457. What is the principle on which the Forcing-pump acts? 458. De- 
scribe the form of forcing-pump represented in Fig. 205. 459. Explain its operation. 
When there is no air-chamber, how does the forcing-pump drive out the liquid? 
460. Of what does the Fire-engine consist ? Describe its operation with Fig. 206. 



THE FIRE-ENGINE. 



189 




pivot K, so that one rises as the other j^g^ Fig- 206. 

descends. They are driven up and 
down by brakes attached to the beam 
and worked by a number of men on 
each side. F is the suction-pipe. H 
is the air-chamber, and E a pipe ris- 
ing from it, to which a flexible leather 
hose is attached, so that the stream 
can be turned in any direction. The 
piston D in Fig. 206 is ascending, fol- 
lowed by a stream of water from the 
reservoir below, the valve I leading 
into the air-chamber being closed. 
The piston C, on the other hand, is 
descending ; its lower valve is closed, 
and the water drawn into the cylinder 
during its previous ascent, is now being forced into H, through the open 
valve J. — The most efficient fire-engines are now worked by steam. 

461. The fire-engine is one of the most powerful forms of the forcing- 
pump, since water is being constantly forced into the air-chamber by one 
of the pistons, and the air is violently compressed. — With a good engine, 
worked by steam, a stream can be thrown more than 200 feet high. 

462. The Centrifugal 
Pump.— The Centrifugal Pump 
(Fig. 207) is an instrument 
for raising water by the com- 
bined effect of the centrifugal 
force and atmospheric pres- 
sure. 

It consists of a vertical 
axle, AB, and one or more 
tubes, C, C, fastened to it, 
extending into a reservoir of 
water below, and branching 
off towards the top so as to 
bring their mouths over the 
circular trough D. E is a 
spout for discharging the wa- 




THE CENTRIFUGAL PUMP. 



461. What is said of the power of the fire-engine ? How high can a stream be thrown 
with a good engine ? 462. What forces are brought to "bear in the Centrifugal Pump? 



190 



PNEUMATICS. 



ter from the trough. Near the top and bottom of each 
tube is a valve opening upward. 

463. Operation. — When the pump is to be worked, the tubes are filled 
with water, which is prevented from escaping by the lower valves. A rotary 
motion is then communicated to the tubes by means of a handle attached to 
the axle. The centrifugal force at once acts on the water within, causing it 
to open the valves and rush forth from the mouths of the tubes. As it as- 
cends, a vacuum is left behind it, into which water is driven by atmospheric 
pressure from the reservoir below. Streams are thus kept pouring into the 
trough as long as the rotary motion is continued. 

A large centrifugal pump, worked by steam, has raised no less than 1,800 
gallons a minute to a considerable height. 

464. The Stomach Pump. — The Stomach Pump is an 
instrument for injecting a liquid into the stomach of a poi- 
soned person and withdrawing it, without removing the 
apparatus. The stomach is thus rinsed out, and life is often 
saved. 

Fig. 208. 
C 




THE STOMACH PTTMP. 

Fig. 208 represents the stomach pump. A syringe, A, 
is screwed into a cylindrical box, B, where it communicates 
with a short metallic tube. This tube leads on either side 
into a bulb, which is connected with a tube of india rubber. 
Each bulb contains a movable circular valve of metal, which 
fits either extremity, and may be made to close either by 
raising the opposite side of the instrument. 

Operation. — To work the pump, turn the syringe so as to depress C and 
elevate D ; and then introduce the tube F into the patient's stomach, and E 
into a basin of warm water. The metallic valves fall to the lowest part of 



Of what does the centrifugal pump consist? 463. What is its mode of operation } 
What has been effected with a large centrifugal pump worked by steam? 464. For 
what is the Stomach Pump used ? Describe its parts. How is it worked ? 



EXAMPLES FOR PRACTICE. 191 

their respective bulbs, which brings them directly opposite where they are in 
the Figure. Now draw out the handle of the syringe. A vacuum is pro- 
duced ; and the warm water, under atmospheric pressure, rushes up to fill 
it, all communication with F being cut off by the valve. The syringe being 
thus charged, the handle is pressed back, and the water, prevented from re- 
turning into E by the valve, is forced through F into the stomach. Without 
removing the india rubber tube from the stomach, now turn the instrument, 
so as to raise the side C and depress D, as shown in the Figure. The metal- 
lic valves are thus thrown to the opposite extremities of their bulbs, and by 
working the syringe with them in this position, the contents of the stomach 
are drawn off and discharged into the basin. The syringe is thus always 
charged through the depressed tube and emptied through the elevated one. 

465. The consideration of the steam-engine, the great- 
est of pneumatic machines, is deferred till we shall have 
treated of the mode of generating steam by heat, a subject 
which belongs to Pyronomics. 

EXAMPLES FOR PRACTICE. 

1. (See § 398.) Under a pressure of one atmosphere, a body of oxygen fills 24 

cubic inches, and its specific gravity is 1.111. What space will it occupy, 
and what will be its specific gravity, under a pressure of three atmos- 
pheres ? 

2. Some hydrogen, by a pressure of 20 pounds to the square inch, is forced 

into a space of one cubic foot. How great a pressure will compress k 
into half a cubic foot, and how will its density then compare with what 
it was before ? 
8. Into what space must we compress 10 cubic inches of air, to double its 
elastic force ? 

4. (See §401.) What is the weight of 600 cubic inches of air? What is the 

weight of the same bulk of water ? 

5. A vessel, full of air, weighs 1,061 grains ; exhausted, it weighs but 1,000 

grains. How many cubic inches does it contain ? 

6. (See § 414.) What is the downward atmospheric pressure on the roof of a 

house containing 115,200 square inches ? What is the upward atmos- 
pheric pressure on the same roof? 

7. What amount of atmospheric pressure is supported by a boy whose bod_\ 

contains 1,000 square inches of surface? 

8. (See § 408.) When the mercury in the barometer stands at 29 inches, at 

what height will a column of water be supported by the atmosphere? 

[Solution.— The specific gravity of water is 1 ; that of mercury, 13.568. 
A column of water will be supported at the height of 29 X 13.563 inches.'] 

9. When the atmosphere supports a column of water 32 feet high, how high 

a column of mercury will it support? 

10. (See Fig. 183.) How far above the earth's surface would the mercury 
stand only two inches high in the barometer ? 



192 PYRONOMICS. 

CHAPTER XIII. 

PYRONOMIOS. 
466. Pyeonomics is the science that treats of heat. 

What Heat is. 

46*7. Heat, a Force. — When we approach a fire, we 
experience a sensation which we call Heat. That which 
produces the sensation is also called Heat. 

In the latter sense, Heat is an immaterial force , result- 
ing from vibrations in the molecules, or atoms, of matter. 
The more rapid the vibrations, the greater the heat. 

When, therefore, we speak of heat as diffusing itself, carried, conducted, 
or transmitted, we use these terms for the sake of convenience, not meaning 
that any material substance passes, but that vibrations in the atoms of one 
body, or part of a body, have been communicated to the atoms of another. 

468. Cold is a relative term, implying a greater or less 
deficiency of heat. 

469. Temperature. — The Temperature of a body is 
the amount of heat that it contains. 

We can not always judge correctly of a body's temperature by the sen- 
sation it produces when we touch it. In the same room, for instance, are 
a bar of iron and a piece of cloth ; they must be of the same temperature, 
but the iron is cold to the touch while the cloth is not. This is because 
the iron carries off the heat more rapidly from the part that touches it. 
So, if one hand be cold and the other warm, a substance which to the^ 
former seems hot, to the latter may appear just the reverse. 

470. The Dynamic Theory. — The old theory respect- 
ing heat represented it as a kind of matter, — an exceed- 
ingly subtile substance residing between the atoms of bod- 
ies, — whose particles repelled each other, while they had 
a strong affinity for ordinary matter. 

466. WhatisPyronomics? 467. What is Heat ? What do we mean, when we speak 
of heat as carried off <>r transmitted ? 468. What is Cold ? 469. What is meant by 
the temperature of a body ? Show that we can not judare of a body's temperature by 
the sensation we experience when we touch it. 470. What was the old theory re- 
specting heat? Give the Dynamic Theory. 471. What is meant by the correlation 



NATURE AND SOURCES OF HEAT. 193 

The Dynamic Theory, now universally received, denies 
that heat is matter, and regards it as a force, indicated by 
certain effects, and resulting from rapid vibrations of the 
molecules of ordinary matter. 

471. Correlation or Forces. — According to the dy- 
namic theory, heat is but one mode of force originating in 
molecular motion ; light, electricity, magnetism, and chem- 
ical affinity, are other modes. Each of these forces is con- 
vertible into each of the others, — may produce the rest 
and be produced by them. Light, for instance, is accom- 
panied with heat, and a high degree of heat with light. 

472. Conservation of Forces. — According to the 
dynamic theory, force, like matter, was created in the 
beginning, and like matter is indestructible. One kind 
of force is constantly being transformed into another, but 
no portion of force is ever lost ; the whole quantity in the 
universe is unalterable. 

Sources of Heat. 

473. The principal sources of heat are the Sun, Chemi- 
cal Action, Mechanical Action, and Electricity. 

474. The Sun, a Source of Heat. — The Sun is the 
great source of heat and light to the earth. The rapid 
vibrations of the atoms on the sun's surface, according to 
the dynamic theory, communicated to the ether and prop- 
agated through it for millions of miles, reaching our eyes 
produce the sensation of light, and striking on various 
bodies produce a more rapid vibration of their atoms, or 
heat. The stars, which are simply very distant suns, pro- 
duce the same effects, but in a much less degree. 

It is hard to account for the undiminished supply of heat on the sun's 
surface. A late theory attributes it to a stream of countless meteors con- 
stantly pouring into the solar atmosphere, by friction against which they 
are set on fire, like meteors entering the atmosphere of the earth. 

475. The heat at the sun's surface is supposed to be 
more intense than any with which we are acquainted. By 

of forces ? 472. What is the conservation of forces ? 473. "What are the chief sources 
of heat ? 474. What is the srreat source of heat to the earth ? How does the sun 
produce light and heat ? 475. How great is the heat at the sun's surface supposed 

IS 



194 



PYRONOMICS. 



the time it reaches us, modified by the immense distance it 
has traversed, it is just sufficient to warm the earth into 
fertility. 

The sun does not heat all parts of the earth alike. This is because its 
rays strike some portions perpendicularly and others obliquely. The per- 
pendicular rays are absorbed more than the oblique ones, and therefore pro- 
duce a greater degree of heat in the parts on which they strike. For the 
fame reason, it is hotter about noon than any other time of day, the sun 
|)eing then more directly over head. 

The variety of productions in different parts of the earth is owing to the 
difference in the amount of heat received from the sun. The trees and plants 
of the tropics are quite different from those of the temperate regions, and 
these again are unlike those of cold climates. In the far north and south, so 
little heat is received that vegetation entirely ceases. 

Fig. 209. 476. The sun's heat may be increased 

by collecting a number of its rays into 
one point called a Focus. This may be 
done with a convex lens, or glass of the 
shape represented in Fig. 209. With 
such a lens, three feet in diameter, the 
metals have been melted. 




A similar effect may be produced with concave 
mirrors, so arranged as to reflect the rays that strike 
them to one and the same focus. When the Romans 
were besieging Syracuse, 213 b. c, Archimedes is 
said to have used a number of metallic mirrors with 
such effect as to set fire to their fleet. The experi- 
ment has been repeated in modern times. Buffon, 
with a combination of 168 mirrors, showed that 

tarred planks could be set on fire at a distance of 150 feet, and that at 60 feet 

silver could be fused. 

477. Heat below the Earth's Surface. — The sun's heat, 
even when it falls perpendicularly on the surface, does not 
penetrate into the earth farther than 100 feet. Beyond 
this depth, all the heat that is felt, comes, not from the 
sun, but from the interior of the earth. 



to be? Why is it less intense when it reaches us? Why does not the sun heat all 
parts of the earth alike ? To what is the variety of productions in different parts of the 
earth owing ? 476. How may the sun's heat be increased ? In what other way may a 
similar effect be produced ? What did Archimedes accomplish with a number of me- 
tallic mirrors? Give an account of Buffon's experiment. 477. What is the greatest 



SOURCES OF HEAT. 195 

As we descend below the earth's surface, the temperature increases about 
one degree for every 55 feet. At this rate, water would boil at a depth of 
less than two miles, and at 30 miles all known substances would be melted. 
It is thought, therefore, that the great mass of the interior of the earth is in 
a state of fusion. The discharge of melted earthy matter, called lava, during 
the eruption of volcanoes, goes to prove this ; while the hot springs in differ- 
ent parts of the world (particularly numerous in Iceland) show that a high 
temperature prevails at no very great depth. At the surface this internal 
heat is not perceptible, because the outer crust of the earth is a bad conductor. 

478. Chemical Action, a Source of Heat. — Chemical 
affinity is the force by which two dissimilar bodies tend 
to unite and form a new substance having different prop- 
erties from either; when they thus unite, we say that 
Chemical Action takes place. Such action is always accom- 
panied with an increase of temperature ; the atoms, flying 
to enter into new combinations under the influence of chem- 
ical affinity, are set in violent motion, and heat is the result. 
The heat generated is sufficient, in some cases, to ignite 
inflammable substances. Thus, a drop of sulphuric acid 
will set fire to a mixture of sugar and chlorate of potassa. 

479. Combustion. — One of the commonest processes in 
which chemical action is exhibited, is Combustion, or Burn- 
ing. This is the great source of artificial heat, as the sun 
is of natural heat. 

Combustion is nothing more than a chemical union of the oxygen of the 
air with the combustible body or some of its elements. The heat which 
accompanies the chemical action renders luminous the gases or vapors 
produced ; and hence what we call Flame. The rise of temperature is pro- 
portioned to the rapidity with which the chemical union takes place ; and 
this depends in a great measure on the amount of oxygen supplied. 

If we wish to make a fire hotter, we have only to bring more air in con- 
tact with the fuel. This may be done with a bellows, or in the case of grates 
with a blower. To fill the vacuum produced by the ascent of the heated air 
through the chimney, cold air must enter ; by putting on the blower, we pre- 

distance to which the sun's heat penetrates? Beyond this depth, whence is the heat 
derived? Descending below the earth's surface, at what rate does the temperature 
Increase ? At what depth would water boil ? How great would the temperature be 
8t a depth of 125 miles ? In what state is the interior of the earth supposed to be ? 
What phenomena support this opinion ? 478. When does Chemical Action take place ? 
With what is chemical action always accompanied ? Give an example. 479. In what 
common process is chemical action exhibited? What is Combustion ? What is the 
cause of flame ? To what is the rise of temperature proportioned ? What must be 



196 PYKONOMICS. 

vent it from entering anywhere except at the bottom of the grate, and cause 
what does enter to pass through the ignited coals, thus increasing their sup- 
ply of oxygen. 

480. Animal Heat. — To Chemical Action is attributable 
Animal or Vital Heat, — that is, the heat generated in all 
organic beings that possess life. 

Different living creatures have different degrees of ani 
aaal heat. Birds have the most ; beasts come next ; then 
iish and insects. In the same class of animals, however, the 
amount of vital heat is nearly uniform ; and under ordinary 
circumstances it remains the same, whether the surround- 
ing medium be warm or cold. Other things being equal, 
the heat of the human body is as great in winter as in sum- 
mer, in the frigid as in the torrid zone. We do not feel 
equally hot, to be sure ; but, as already explained, we must 
not judge of temperature by our feelings. 

481. Animal heat is produced "by a slow combustion. When we breathe, 
air is taken into the lungs. Penetrating the delicate vessels on their sur- 
face, the oxygen of the air enters the blood, and is carried by it, through 
the heart, to minute capillaries in the different organs. Here it unites 
chemically with particles of carbon from the tissues, and heat is generated. 
Carbonic acid, the new substance formed by the chemical action, is carried 
back by the blood to the lungs, and there discharged into the air to be 
exhaled, while a fresh supply of oxygen is obtained for a repetition of the 
process. 

As in combustion, whatever increases the supply of oxygen increases the 
animal heat. Eunning or bodily exertion of any kind makes us hotter, be- 
cause it quickens the circulation of the blood, obliges us to breathe faster, 
and thus brings more air (and consequently more oxygen) into the lungs. 

The carbon consumed comes from the food we eat. Greasy food gen- 
erates it most plentifully. In winter, therefore, when we need an abundance 
of carbon, we eat meat more freely than in summer, when we seek to reduce 
our vital heat as much as possible. So, the inhabitants of cold regions con- 
sume more greasy food than those of warmer climates. The Esquimaux 
thrive on fish-oil and seals' fat, which to the people of the tropics would 
be neither palatable nor wholesome. 

done, if we wish to make a fire hotter? 480. What is Animal or Vital heat? To 
what is it attributable? What is said of animal heat in different living creatures? 
In the same class of animals ? Does it differ in different seasons ? 481 How is ani- 
mal heat produced ? How is animal heat increased ? Give examples. Whence 
comes the carbon consumed ? What sort of food generates carbon most plentifully ? 
What follows, with respect to our diet at different seasons? How does the diet of 
•he inhabitants of cold regions compare with that of tropical nations ? 482. What 1p 



SOURCES OF HEAT. 197 

482. Mechanical Action, a Source of Heat. — Me- 
chanical Action is a familiar source of heat. Under this 
head are embraced Friction or Rubbing, Percussion or 
Striking, and Compression. All mechanical motion, which 
is a motion of masses, transferred to the atoms of matter, 
is exhibited as heat. 

483. Heat from Friction. — Touch a row-lock, in which 
an oar has been rapidly plying, or a gimlet that has just 
been vigorously worked, and you will feel the heat pro- 
duced by friction. Rub a metallic button to and fro on a 
dry board, and you will soon make it so hot that you can 
not bear your finger on it. By drawing a match across a 
rough surface, you develop heat enough to ignite it. By 
rubbing two pieces of ice together, in a freezing tempera- 
ture, heat is generated in sufficient quantities to melt them. 

Machinery has been ignited by the rubbing of its parts on each other. 
Savages kindle a fire by rubbing two dry sticks violently together. In bor- 
ing a brass cannon, immersed in water by way of experiment, sufficient heat 
has been generated to boil the water in two hours and a half. The friction 
of two large iron plates has even been employed as a practical source of 
heat. The friction of fluids also produces heat, but in a much less degree. 

484. Heat from Percussion. — By striking flint and steel 
together, we develop sufficient heat to ignite the minute 
fragments broken off, and produce sparks. In like manner, 
the hammer of a gun, descending on a percussion-cap, sets 
fire to the fulminating mixture of which the cap is made, 

A nail may be made red-hot by hammering it rapidly on an anvil. Be- 
fore lucifer matches were invented, blacksmiths used to ignite sulphur 
matches and kindle their forge-fires with a nail hammered to a red heat. 

By violent and quick compression, a body of air can be heated suffi- 
ciently to ignite tinder. This is done with the Fire Syringe (Fig. 210). In 
the extremity of the piston is a small cavity, in which some tinder is placed. 
When the piston is driven rapidly down, the air in the barrel is compressed, 



the third source of heat? What are included under ttris head? 483. State some 
familiar cases in which heat is produced by friction. What is sometimes the 
effect of friction on machinery ? How do savages kindle their fires ? How great 
a heat has been produced by boring a brass cannon ? How has friction been turned 
to practical use? What is said of the friction of fluids? 484. Give some famil- 
iar examples of the production of heat by percussion. How did blacksmiths for- 
merly kindle their forge-fires ? Describe the Fire-syringe, and the experiment per- 



£98 PYB0N0MICS. 

Fig. 210. heat is generated, and on withdrawing the piston the tinder 
will be found ignited. 

485. Mechanical Equivalent of Heat. — As 
mechanical force is convertible into heat, so heat 
is convertible into mechanical force. The amount 
of heat necessary to raise the temperature of 1 lb. 
of water one degree, would, if applied mechani- 
cally, raise it 772 feet high. This is expressed 
briefly by saying that the mechanical equivalent 
of heat is 772 foot-pounds. 

486. Electeicity, a Source of Heat. — Elec- 
tricity is sometimes attended with intense heat. 
Lightning, for instance, sets fire to trees and 
houses, and melts metallic bodies that it strikes. 
The heat produced by the galvanic battery ig- 
nites or fuses every known substance. 



Diffusion of Heat* 

487. Heat tends to diffuse itself equally among bodies 
of different temperature ; that is, every vibrating atom 
tends to communicate its own rate of vibration to other 
atoms or to the ether, and has its own motion lessened by 
the amount of motion thus communicated. 

488. Heat is diffused in three ways : — 

. 1. By Conduction, when the molecular vibrations are 
transmitted directly from one molecule to another adjacent 
to it. If one end of a poker is placed in a fire, the other 
becomes heated by Conduction. 

2. By Convection, when atoms in energetic vibration 
are actually conveyed from one part of a body to another, 
in sensible masses, others taking their place, and a circula- 
tion being thus established. Water placed over a fire is 
heated by Convection. 



formed with it. 485. What is the mechanical equivalent of heat, and what is meant 
by the expression ? 486. What is the fourth source of heat ? Give examples. 487. 
What is the tendency of heat ? 488. In how many ways is heat diffused ? Name, 



DIFFUSION OF HEAT. 199 

3. By Radiation, when the molecular vibrations are 
transmitted from one body to another not in contact with 
it, by means of undulations excited in an intervening me- 
dium. A joint of meat placed before the fire is roasted by 
Radiated Heat. 

489. Conduction. — Some substances allow heat to pass 
freely through their particles ; others do not. The former 
are called' Good Conductors of heat ; the latter, Bad Con* 
ductors, or Non-conductors. 

As a general rule, dense solids are good conductors of 
heat ; porous and fibrous solids, as well as liquids, gases, 
and vapors, are bad conductors. 

490. The Conductometer. — The metals Fig. 211. 
are all good conductors of heat, but some 
are better than others. This is shown by 
the Conductometer, Fig. 211. 

The conductometer consists of a circular plate of 
brass, in the outer edge of which are inserted rods of 
different metals, of the same size and length, each 
having a small cavity in its extremity for holding a 
piece of phosphorus. When the plate is brought over 
the flame of a lamp, the heat passes along the different 
rods and ignites the pieces of phosphorus, but not all the conductometer. 
at the same time. It first reaches the end of the rod 

that is the best conductor ; and thus the order in which the pieces of 
phosphorus take fire indicates the order in which the metals that the rod>* 
are made of rank as conductors of heat. 

491. Conducting Power of Different Substances. 
— Silver is the best conductor of heat known. The con- 
ducting power of this metal being taken as 100, that of 
some other metals, according to Tyndall, is as follows : — 




Tin 15 I Platinum , 

Iron 12 German silver , 

Lead 9 Bismuth , 



Copper 74 

Gold 53 

Brass 24 

Geld, once regarded as the best conductor, is now ranked far below 
■silver. 

describe, and give an example of each. 489. What are Good Conductors of heat ? 
What are Bad Conductors ? What substances are good conductors of beat, and what 
not? 490. How do the metals rank in conducting power? Describe the Con- 
ductoraeter, and its mode of operation. 491. Among the metals, Avhat is the best 
conductor ? Tbe nest ? The next ? Which is the better, iron or lead ? How may 



200 PYRONOMICS. 

A- silver spoon containing water, with a piece of muslin wrapped smoothly 
around it, may be held in the flame of a lamp till the water boils without the 
muslin's burning, so rapidly does the metal carry off the heat. 

492. Wood is a bad conductor of heat. A log blazing at one end may be 
handled at the other without inconvenience. Hence metallic tea-pots, sauce- 
pans, &c, are often provided with wooden handles. Dense wood and coal 
are better conductors than porous wood. This is one reason why they are 
harder to kindle ; they conduct the heat away before a sufficient amount is 
oollected in them to produce combustion. Earthen- ware of all kinds ranks 
far below the metals in conducting power. 

493. Fibrous substances, like wool, hair, and fur, are bad conductors. 
The finer and closer their fibres, the less their conducting power. Thus we 
see why Providence has clothed the animals of cold climates with a shaggy 
covering, from which those of the tropics are free ; and why the coats of 
many animals in temperate regions change with the seasons, being closer and 
longer in winter, thinner and shorter in summer. 

494. The best non-conductors among solids are straw, saw-dust, pow- 
dered charcoal, and plaster of paris. Recourse is had to these articles when 
it is desired to protect an object from extremes of temperature. Straw is 
bound round tender plants in winter, to prevent their warmth from being 
drawn off. It is also used for thatching the roofs of houses, preventing the 
external heat from entering in summer, and the heat within from being with- 
drawn in winter. Ice shipped to warm climates is packed in saw-dust, to 
keep out the heat of the atmosphere. For the same reason, the hollow apart- 
ments that constitute the sides of refrigerators are filled with powdered char- 
coal. Plaster of paris is used for filling in the sides of fire-proof safes. So 
impervious to heat does it render them that they may be exposed to flames 
for hours without injury to the papers within. 

495. If we bare our feet, and place one of them on a 
carpet and the other on oil-cloth, the latter feels much 
colder than the former. This is not because the oil-cloth 
is colder than the carpet, for being in the same room their 
temperature must be the same ; but oil-cloth is a good con- 
ductor, whereas carpet is not. A good conductor, brought 
in contact with the body, carries oif our animal heat and 
makes us feel cold. A bad conductor, on the other hand, 
prevents our animal heat from escaping. Hence the diifer- 

fhe conducting power of silver be proved ? 492. Why are metallic tea-pots often pro- 
vided with wooden handles ? Why is dense wood hard to kindle ? How does earthen- 
ware rank in conducting power? 493. How do fibrous substances rank? As re- 
gards the coats of animals, how is the goodness of Providence shown? 494. What 
are the best solid non-conductors ? For what are these substances severally used, and 
what is the effect in each case ? 495. If we bare our feet, and place one on a carpet 
ind the other on oil-cloth, what do we feel ? Explain the reason of this. Of the 



DIFFUSION OF HEAT. 



201 



ence of warmth in different kinds of clothing. That fabric 
feels the warmest, which is the worst conductor. 

Of the materials used for clothing, wool is the worst conductor and linen 
the best ; cotton and silk rank between the two. Linen is therefore the most 
comfortable fabric for summer clothing, and woollen for winter. A linen 
under-garmeut is cooler than a silk or muslin one, and these in turn are 
much cooler than flannel. 

496. The heat of our bodies is generally greater than that of the atmos- 
phere surrounding them. If we were placed in an atmosphere warmer than 
our bodies, woollen would be the coolest dress that could be worn, because, 
being a bad conductor, it would not transmit the external heat. Hence fire- 
men and others exposed to a high degree of heat, always wear flannel. 
Hence, also, a blanket is wrapped round ice, to keep it from melting. 

497. Conducting Power of Liquids. — Liquids (except 
mercury, which is a metal) are very bad conductors of heat. 
This may be shown by several experiments. 

Freeze some water in the bottom of a tube, -pi*. 212. 

and on the ice pour some more water. Inclining 
the tube, apply the flame of a lamp to the liquid 
till it boils. The ice remains for a long time un- 
melted. If mercury is used instead of water, the 
ice begins to melt almost immediately on the ap- 
plication of heat. 

Again, in a funnel-shaped glass vessel (repre- 
sented in Fig. 212) fix a thermometer, or instru- 
ment for measuring heat, with its bulb uppermost. 
Cover the bulb with water to the depth of half an 
inch ; then pour on some ether, and set fire to it. 
The burning of the ether generates a great heat ; 
yet the thermometer, only half an inch below it, 
indicates little or no increase of temperature. 

498. Conducting Power of Gases 
and Vapors. — Gases and vapors are 
still worse conductors of heat than liquids. The less their 
specific gravity, the less appears to be their conducting 
power. 

499. Air is one of the worst conductors known. If we 




materials used for clothing, which is the worst conductor? Which, the best ? How 
do cotton and silk rank ? What fabric, then, is the most appropriate for summer 
wear, and what for winter? 496. Why do firemen wear flannel ? Why is a blanket 
wrapped round ice ? 497. How do liquids rank in conducting power? Prove that 
water is a bad conductor. Prove it by an experiment with the apparatus represent- 
ed in Fig. 212. 498. How do gases and vapors rank in conducting pow^r? 499. What 



202 



PYRONOMICS. 



could keep a body of air perfectly still, it would take a long 
time for heat applied to one portion of it to be transmitted 
throughout the whole. 

In summer, when there is no breeze, we feel oppressively warm, because 
the air does not carry off the heat generated within us. Fanning cools us, 
because it drives off the air heated by contact with our bodies and brings up 
a fresh supply, which, after withdrawing more or less heat, is in turn driven 
away. In this case it will be observed that the heat is carried off by convec- 
tion, and not by conduction. If air were a good conductor, it would soon 
take so much heat from animals and plants that their vital action could not 
make up the deficiency, and they would be chilled to death. 

Closed cellars are cooler than the surrounding air in summer, and warm- 
er in winter. If air were a good conductor, this would not be the case. As 
it is, the doors being kept closed, currents of air are excluded ; and, since 
heat passes very slowly from particle to particle, extremes of temperature 
without are not felt within. 

It is the air in fibrous and porous solids that makes them bad conductors. 
Drive out this air by compression, and you increase their conducting power. 
Let wool, or cotton, for instance, be twisted into rolls, and it will carry off 
heat faster than it did when loose. Accordingly, clothing that allows some 
air to remain in contact with the body is warmer than that which fits very 
tight. So, double sashes and double doors, confining a body of non-con- 
ducting air, protect apartments from extremes of heat and cold. 

500. The uses of air as a non-conductor are seen in the operations of na- 
ture. Filling the pores and interstices in the bark of plants, it protects the 
tender parts within from sudden falls of temperature. In cold climates, vege- 
tation is further protected by snow, which, owing to the air imprisoned 
among its particles, is a very bad conductor. A mantle of snow on a field 
has very much the same effect that a covering of wool would have. Hence 
we are told in Scripture that God " giveth snow like wool ". — The Esquimaux 
shield themselves from the excessive cold of their climate in huts of snow. 

501. Coxvection. — Fluids, as we have just seen, are 
bad conductors, but they are readily heated by convection. 
Heat being applied beneath, the lower particles become 
expanded and rarefied. They therefore ascend, carrying 
up their heat, while cooler and heavier particles from above 



is said of the conducting power of air? Why do we feel oppressively warm in sum- 
mer, when there is no breeze? What is the effect of fanning ? If air were a good 
conductor, what would be the consequence to animals and plants ? Why are closed 
2ellars exempt from extremes of temperature ? What makes fibrous and porous sol- 
ids bad conductors ? Prove this. Compare the warmth of loose clothing with that 
which fits very tight. On what principle do double sashes operate ? 500. Show the 
uses of air as a non-conductor in the economy of nature. What is the effect of snow ? 
What use is made of it by th^ Esquimaux? 501. How are fluids readily heated? 



DIFFUSION OF HEAT. 203 

take their place. This process is repeated till heat is dif- 
fused throughout the whole, — not conducted from one sta- 
tionary particle to another, but actually conveyed by the 
particles receiving it. 

The process of convection is exhibited when water is set over a fire to 
boil. The particles soon begin to move, as may be shown by throwing in 
some powdered amber, which is seen to rise and descend, more and more 
rapidly as the temperature increases. Heat is thus diffused throughout the 
whole body of liquid, till ebullition, or boiling, commences. 

502. In cooling, this process is reversed. The particles at the top yield 
their heat to the air in contact with them. Being thus made heavier, they 
descend, while warmer and lighter particles take their place. The greater 
the surface exposed to the air, the sooner the liquid loses its heat ; hence we 
pour our tea into a saucer, to cool it. 

503. To heat a body of liquid by convection, the fire must be applied be- 
neath. A pot of water can not be made to boil by a fire kindled on its lid. 
The particles at the top may be heated, but they will remain there on ac- 
count of their superior lightness, and there will be no diffusion of heat. 

504. Thin liquids, like water, are heated and cooled more quickly than 
thick ones, like tar, because their particles move more freely among them- 
selves, and thus diffuse heat more readily. 

505. Heat is diffused through gases and vapors, as 
through liquids, by convection. Heated air, like heated 
water, ascends, carrying its heat with it. Consequently, to 
make the temperature of a room uniform, a fire-place should 
be set as near the floor as possible. — With the same tem- 
perature, we feel colder on a windy day than on a still one ; 
because the heat is more rapidly withdrawn from our bodies 
by the fresh currents of air constantly brought in contact 
with them. 

506. Solids can not be heated by convection, because 
their particles cohere. 

507. Radiation. — A body not in contact with the source 
of heat can not be heated by conduction or convection. If 
it receives heat, it is by a third process, called Radiation. 

Describe the operation. In what familiar process is convection exhibited ? Describe 
the process of boiling. 502. Describe the process of cooling. 503. To heat a liquid, 
where must the fire be applied ? Why can not a pot of water be made to boil by a fire 
kindled on its lid ? 504 What kind of liquids are heated and cooled most quickly ? 
Why ? 505. What, besides liquids, are heated by convection ? Where should a fire- 
place 1»e set, and why ? Why do we feel colder on a windy day than on a still one ? 
506. Can solids be heated by convection? Why not? 507. What bodies are heated 



204 PYRONOMICS. 

If we place our hands under a fire in a grate, we at once feel a sensation 
of heat. This heat can not reach our hands by conduction, for air is a bad 
conductor, — nor by convection, for heated currents ascend. It is transmitted 
in rays sent forth from the fire through the intervening space. Heat thus 
diffused is called Radiant Heat. All the heat that we receive from the sun, 
and much of that from fire, is radiant heat. 

508. All substances radiate heat, but not equally well 
Much depends on the character of the surface. Rough and 
dull surfaces radiate better than smooth and bright ones. 

Lamp-black is the best radiator known. Rating its ra- 
diating power at 100, that of crown-glass is 90 ; black lead, 
75 ; tarnished lead, 45 ; clean lead, 19 ; bright metals gen- 
erally, 12. The radiating power of metals is increased by 
scratching their surface, or letting them become tarnished. 

509. A heated body confined in a covered vessel parts with its heat more 
or less rapidly according to the radiating power of the vessel containing it. 
For tea-pots, therefore, bright silver is preferable to earthen-ware, because it 
is a worse radiator and keeps the tea warm for a longer time. Stoves, on 
the contrary, should be made of a good radiator, so that the heat of the fire 
may be freely diffused. Cast-iron is better for this purpose than sheet-iron, 
because its surface is rough ; the radiating power of both is increased by 
rubbing in black lead. When heat is to be conveyed from one room to an- 
other, a pipe should be used of bright tin, which is a bad radiator and pre- 
vents the escape of heat by the way. 

The atmosphere receives its heat, not directly from the sun, but by radia- 
tion from the earth ; hence, as we ascend from the earth's surface, the heat 
diminishes. 

510. Law of Radiant Heat. — Radiant heat diminishes 
in intensity as the square of the distance from the radiating 
body increases. 

A body 10 feet from a fire will receive from it only i/ioo of the heat that a 
body 1 foot from it receives. 

511. Radiant heat, striking different bodies, is reflected 

by radiation ? What is heat diffused by radiation called ? Give a familiar example 
of radiant heat. 508. By what is a body s radiating' power affected ? What surfaces 
radiate heat the best? What is the best known radiator? Eating the radiating 
power of lamp-black at 100, what is that of crown-glass? Black lead? Tarnished 
lead ? Clean lead ? Bright metals generally ? How may the radiating power of the 
metals be increased ? 509. Why is bright silver preferable to earthen-ware for tea- 
pots ? Of what should stoves be made ? When heat is to be conveyed from one room 
to another, what should be employed? Why? How does the atmosphere receive 
its heat ? What follows ? 510. State the law of radiant heat. Give an example. 



DIFFUSION OF HEAT. 



205 



by some, absorbed by others, and transmitted by a third 
class. 

512. Reflection of Radiant Heat. — Radiant heat is re- 
fleeted by polished and light-colored surfaces. Polished 
gold reflects about three-fourths of the radiant heat it re- 
ceives, and looking-glass about one-fifth ; whereas metallic 
surfaces blackened reflect only one-twentieth. 

513. White and light-colored clothes are worn in summer, because thej 
reflect heat. For the same reason, it is harder to heat water in a new tin ves- 
sel than in one that has been blackened over the fire. 

514. The reflection of radiant heat may be illustrated with the apparatus 
represented in Fig. 213. A and B are concave metallic mirrors, highly pol- 

Fig. 213. 





ished. In the focus of A is placed a red-hot ball C. This ball radiates heat 
in all directions, and some of its rays strike the mirror A, from which they 
are reflected in parallel lines to B. By B they are again reflected and brought 
to a focus at D, where a thermometer indicates a rise of temperature. Suffi- 
cient heat may thus be concentrated at D to set fire to phosphorus or gun- 
powder. 

515. When radiant heat is reflected by a plane surface, 
the angle of reflection (see § 96) is always equal to the an 
:;le of incidence. If it strikes the surface perpendicularly, 
it is reflected perpendicularly, back to the radiating body, 
if the line in which it approaches the surface forms an angle 



631. When radiant heat strikes different bodies, what becomes of it? 512. By what 
Purfaces is radiant heat reflected ? What portion does polished gold reflect? Look- 
fng-glass ? Metallic surfaces blackened ? 513. Why are light-colored clothes worn in 
summer ? In what sort of a vessel is it hardest to heat water ? 514. Illustrate the 
reflection of radiant heat with Fig. 213. How much heat may be concentrated with 
this apparatus ? 515. When radiant heat is reflected, to what is the angle of reflection 



206 PYRONOMICS. 

with the perpendicular, it glances off at an equal angle on 
the other side. 

516. Absorption of Radiant Heat. — Radiant heat is 
absorbed by dull and dark-colored surfaces. Good reflec- 
tors are bad absorbents and radiators ; bad reflectors are 
good absorbents and radiators. 

Of the colors, black is the best absorbent of heat, and 
violet the next best ; white is the worst, and yellow next to 
the worst. 

Lay two pieces of cloth, one white and the other black, on a snow-bank, 
in the sunshine. Under the black piece, which absorbs the heat that strikes 
it, the snow melts rapidly ; not so under the white cloth, for by it the heat is 
reflected. Dark-colored clothing is therefore best adapted to winter. 

Dark mould absorbs the sun's heat; hence one cause of its fertility. 
White sand reflects the hot rays ; hence it burns our faces when we walk 
over it in summer. Hoar-frost remains longer in the morning on light than 
dark substances : this is because light colors reflect the sun's heat, while 
dark colors absorb it, and thus melt the hoar-frost, which is nothing more 
than frozen dew. 

517. Transmission of Radiant Heat. — Transparent sub- 
stances, or such as allow light to pass through them, for 
the most part transmit heat also. The sun's rays, for in- 
stance, falling on the atmosphere of the earth, which is a 
transparent medium, are transmitted through it to objects 
on the surface. More or less heat is absorbed in the act 
of transmission. 

518. Substances that transmit heat freely are called Di- 
a-ther'-ma-nous. Those that absorb the greater part and 
transmit little or none are called A-ther'-ma-nous. 

519. All transparent substances are not diathermanous. Water, for ex- 
ample, which offers but little obstruction to rays of light, intercepts nearly 
all the heat that strikes it. Alum is another instance in point. 



equal ? 516. By what surfaces is radiant heat absorhed ? What is said of good reflect- 
ors? What, of bad reflectors? What color is the best absorbent of heat? What, the 
next best? What color is the worst absorbent? What, the next worst ? Prove by 
an experiment the difference in absorbing power between white and black. Why is 
dark-colored clothing best adapted to winter ? What is the difference between dark 
mould and white sand in absorbing power? Why does hoar-frost remain longer in 
the morning on light than dark substances ? 517. What substances, for the most part, 
transmit heat ? Give an example. 518. What are Diathermanous substances? What 
are Athermanous substances ? 519. Name a transparent substance that is not dia- 



EFFECTS OF HEAT. 207 

All diathermanous substances are not transparent. Quartz, though it may 
intercept light almost entirely, transmits heat quite freely. 

As a general rule, the rarer transparent substances, such as gases and 
vapors, transmit heat the best ; the denser ones, such as rock-crystal, trans- 
mit it the least freely. The farther the rays have to pass through a given 
substance, the more heat is intercepted. 

Effects of Heat. 

520. The effects of heat are five in number: Expansion, 
which changes the size of bodies ; Liquefaction and Vapor- 
ization, which change their form ; Incandescence, which 
changes their color ; and Combustion, which changes their 
nature. 

521. Expansion. — Heat expands bodies. 

Atoms in violent vibration urge each other apart, and cause the body 
to which they belong to occupy a greater space. Heat, therefore, opposes 
cohesion. Solids, in which cohesion is strongest, expand the least under 
the influence of heat ; liquids, having less cohesion, expand more ; gases 
and vapors, in which cohesion is entirely wanting, expand the most. Heat 
converts solids into liquids, liquids into gases and vapors, by weakening 
their cohesion. It turns ice, for example, into water, and water into steam. 

522. Expansion of Solids. — All solids except clay are 
expanded by heat ; but not equally. Of the metals, zinc is 
among those that expand most. Clay is contracted by bak- 
ing, and ever afterwards remains so ; this is supposed to be 
owing to a chemical change produced in it by heat. 

The expansion of solids is illustrated with the apparatus represented in 
Fig. 214. A brass ball is suspended from a pillar, to which is also at- 
tached a ring just large enough to let the ball pass through it at ordinary 
temperatures. Heat the ball with a lamp placed beneath, and it will ex- 
pand to such a degree that it can not pass through the ring. Let it cool, and 
it will go through as before. 

523. A sheet-iron stove in which a hot fire is quickly kindled or put 
out, sometimes makes a cracking noise, in consequence of the rapid ex- 

thennanous. Name a diathermanous substance that is not transparent. As a gen- 
eral rule, what transparent substances transmit heat the best, and what the worst? 
620. State the effects of heat. 521. What is the first of these ? How is it that heat 
expands bodies ? What force does it oppose ? Which expand the most under tha 
influence of heat, solids, liquids, or gases, — and why ? Into what does heat convert 
solids? Into what, liquids? 522. What solids are expanded by heat? What meta! 
is expanded more than most of the others ? What is the effect of heat on clay ? II- 
\nstrate the expansion of solids with the apparatus represented in Fig. 214. 523. Why 



208 



PYRONOMICS. 



Fig. 214. 




pansion or contraction of the metal. A blower 
placed on or taken from a hot fire produces a sim- 
ilar noise for the same reason. New furniture 
standing in the sun or near a fire is apt to warp 
and crack in consequence of the expansive effects 
of heat. 

When boiling water is poured into china cups 
and glass vessels, they often crack. This is be- 
cause the inner surface is expanded by heat 
while the outer is not, china-ware and glass be- 
ing bad conductors. The unequal expansion 
cracks the vessel. Cold water poured on a hot 
glass or stove produces the same effect. On the 
same principle, glass chimneys are apt to crack, 
when brought too suddenly over the flame of a 
lamp or gas-burner. A cut made in the bottom 
with a diamond allows an opportunity for expan 
sion, and prevents the chimney from breaking. 
When a glass stopper becomes fastened in a bottle, it may often be with 

drawn by placing the neck of the bottle in warm water. The neck is ex 

panded before the heat reaches the stopper. 

524. The force with which a body expands when heat- 
ed and contracts when cooling, is very great. In iron 
bridges, therefore, and other structures in which long bars 
of metal are employed, there is danger of the parts' sep- 
arating, unless provision is made for the expansion caused 
by a rise of temperature. The middle arch of an iron 
bridge has been known to rise an inch in the summer of a 
temperate climate. So, when great lengths of iron pipe 
are laid for conveying steam or hot water, sliding joints 
must be used, or the apparatus will burst in consequence 
of the expansion of the metal. 

525. The fact that heat expands bodies and cold contracts them, is often 
turned to practical account. Coopers, for instance, heat their iron hoops, 
and while they are thus expanded put them on casks which they just 
fit. As they cool, they contract and bind the staves tightly together. Tha 



do a sheet-iron stove and a blower sometimes make a cracking noise ? What causes 
new furniture to warp ? What makes glass vessels crack when boiling water is poured 
into them ? When are glass chimneys apt to crack? How may their cracking be 
prevented ? When a glass stopper becomes fastened in a bottle, how may it be with* 
drawn? 524. What is said of the force with which bodies expand and contract? 
What precautions must be taken in consequence ? 525. What practical use is made 
«f the fact that heat expands bodies and cold contracts them ? What ingenious appli- 



EXPANSION. 209 

«rheel-wright fastens the tire, or outer rim of iron, on his wheel in the 
same way. 

The contraction of iron, when cooling, has been ingeniously used for 
drawing together the walls of buildings that have bulged out and threaten 
to fall. Several holes are made opposite to each other in the walls, into 
which are introduced stout bars of iron, projecting on both sides and termi- 
nating at each end in a screw. To each screw a nut is fitted. The bars are 
then heated by lamps placed beneath, and when they have expanded the 
nuts are screwed up close to the walls. As the bars cool, they gradually con- 
tract, and with such force as to bring the walls back to a perpendicular po- 
sition. 

526. Expansion of Liquids. — Liquids, when heated, 
expand much more than solids, but not all alike. Thus 
water, raised from its freezing-point to the temperature at 
which it boils, has its bulk increased one-twenty-second ; 
alcohol, between the same limits, increases one-ninth. 

The higher the temperature, the greater the rate at 
which liquids expand. 

527. In proportion as heat expands liquids, it rarefies 
them, the same quantity of matter being made to occupy 
a larger space. This fact is shown in the process of boil- 
ing, described in § 501. 

528. Water at certain temperatures forms a remarkable 
exception to the general law that liquids are expanded by 
heat and contracted by cold. As it cools down from the 
boiling-point, it contracts, and consequently increases in 
density, till it reaches 39 degrees, or 7 degrees above its 
freezing-point. Below this temperature, it expands. 

The expansion of water in freezing is proved every winter by the burst- 
ing of pipes, pitchers, &c, containing it. The force with which it expands 
is tremendous. An iron plug weighing three pounds and closing a bomb- 
shell filled with water, has been thrown 15 feet by the freezing and expansion 
of the liquid within. Immense masses of rock are sometimes split off by the 
freezing of water which has insinuated itself into minute fissures. 

The expansion and consequent rarefaction of water in freezing, afford a 

cation has been made of the contraction of iron when cooling? Give an account of 
the process. 526. How does the expansion of heated liquids compare with that of 
solids? Compare the expansion of water with that of alcohol. On what does the rate 
at which liquids expand depend ? 527. Besides expanding liquids, what does heat do 
to them? 528. What exception is there to the law that liquids are contracted by 
cold ? How is the expansion of water in freezing proved ? What cases are cited, to 
Bhow the great force with which water expands in freezing ? How does ths expansion 

14 



210 PYEONOMICS. 

striking proof of the goodness of Providence. The great body of a large 
mass of water never becomes cold enough to freeze ; it freezes only on the 
top, where it comes in contact with very cold air. As it is, the ice formed 
on the surface remains there on account of its superior rarity, and protects 
the water below and the fish that inhabit it from further cold. If water 
continued to contract and increase in density as it approached the freezing- 
point, the ice first formed would sink ; the fresh surface exposed to the air 
would in its turn freeze, and another layer of ice would sink ; and this 
would go on till even in a mild winter every body of water would be con- 
verted into a solid mass, and all living things therein destroyed. 

529. Iron, zinc, and several other metals, when cooling down from a 
melted to a solid state, expand like freezing water. This is because the 
particles assume a crystalline arrangement, by which greater interstices are 
left between them. 

530. Expansion of 'Gases and Vapors. — Aeriform bod- 
ies expand equally under a given increase of temperature. 
At the boiling-point of water, their bulk is about one-third 
greater than at the freezing-point. 

531. Fill a bladder with air, tie its neck, and place it before a fire ; the 
heat will soon expand the confined air to such a degree as to burst the 
bladder. 

The popping of grains of corn, the bursting open of chestnuts when 
roasting, and the crackling of burning wood, are caused by the expansion 
of the air within them. Porter-bottles have to be kept in a cool place in 
summer, lest the heat expand the carbonic acid in the porter and break the 
bottles. 

532. Liquefaction. — Heat melts solids. This process 
is called Liquefaction. 

Some solids, such as wax and butter, require but little heat to melt 
them. Others, like metals and stones, melt only at the highest tempera- 
tures that can be produced. Such substances are called refractory. 

Even substances that are liquid at ordinary temperatures may be looked 
upon as melted solids, for they can be reduced by cold to the solid state. 

533. When a solid is converted into a liquid, heat is 
taken from adjacent bodies, to overcome the cohesion of 

of water in freezing exhibit the goodness of Providence ? 529. How do we account 
for the expansion of several of the metals, when cooling down from a melted state? 
530. What is said of the expansion of aeriform bodies ? How great is their expansion, 
when they are raised from the freezing-point to the boiling-point of water ? 531. How 
may we illustrate the expansion of air by heat with a bladder ? What familiar exam- 
ples are given of the expansion of air by heat ? 532. What is Liquefaction ? What 
difference is there in solids, as regards their capability of being melted ? How may 
substances that are liquid at ordinary temperatures be looked upon ? 533. By what 



VAPOEIZATION. 211 

its particles. When a liquid is converted into a solid, the 
heat no longer needed to oppose cohesion is given out. 
Extreme cold is thus modified by the very act of freezing. 

When a solid is rapidly melted, so much heat is absorbed by the liquid 
that intense cold is produced. This is the principle on which freezing mix- 
:ures operate. Ice cream, for instance, is frozen with a mixture of salt and 
snow or pounded ice ; the latter is rapidly melted, and so much heat is ab 
sorbed in the process that the cream is brought to a solid form. 

534. Vaporization. — Heat converts liquids into vapors. 
This process is called Vaporization. 

Heat, applied to a solid, first expands it, then melts it, and finally turns it 
into vapor. Some solids pass at once into vapor, without becoming liquids. 

535. A great degree of heat is not essential to vapori- 
zation. At ordinary temperatures, wherever a surface of 
water is in contact with the air, vapor is formed. This pro- 
cess is known as Spontaneous Evaporation. By its means 
the atmosphere becomes charged with moisture, and clouds 
and dew are formed. The drier the air, and the more it is 
agitated, so as to bring fresh currents in contact with the 
liquid, the more rapidly does evaporation take place. 

5?6. A drop of water let fall on a cold iron moistens its surface ; let fall 
on a very hot iron, it hisses and runs off without leaving any trace of moist- 
ure. In the latter case, the water does not touch the iron at all, but is sep- 
arated from it by a thin layer of vapor into which part of the drop is con- 
verted by the heat radiated from the iron. Laundresses try their irons in 
this way, to see if they are hot enough for use. On the same principle, jug- 
glers plunge their hands into melted metal with impunity, by first wetting 
them. The moisture on their hands is converted into vapor, which keeps the 
seething metal from their skin. 

537. When vapor is formed, heat is consumed in ovet^ 
coming cohesion, and cold is produced. 

Hence when the skin is moistened with a volatile liquid (that is, one that 
readily passes into vapor) like alcohol, a sensation of cold is soon expe- 
rienced. So, a shower or water sprinkled on the floor cools the air in sum- 
merciful provision is extreme cold modified, and how? On what principle do 
freezing mixtures operate? 534. What is Vaporization? What are the successive 
effects of heat on solids ? 535. What is Spontaneous Evaporation ? What are the 
effects of evaporation on the earth's surface ? To what is the rapidity of evaporation 
proportioned ? 536. Explain the principle on which laundresses try their irons. 
What use do jugglers make of this principle ? 537. With what phenomena is the 
formation of vapor accompanied ? Give some examples of cold produced hy the for 



212 



PYRONOMICS. 



mer. — Green wood does not make so hot a fire as dry, because, when the 
moisture it contains is converted into vapor, a large amount of sensible heat 
is absorbed and carried off". 

538. Condensation. — The turning of vapor back into a 
liquid state is called Condensation. 

539. Distillation. — Some substances are converted into 
Vapor at lower temperatures than others. This fact is 
taken advantage of in Distillation. 

Distillation is the process of separating one substance 
from another by evaporating and then condensing it. It 
was known to the Arabians at an early date. Fig. 215 
represents a Still, or apparatus for distilling. 

Fig. 215. 




540. A is a boiler, resting on a furnace. In its head, E, is inserted a pipe, 
b c, which enters the worm-tub, R, and there terminates in a worm., represented 
by the dotted lines. The substance to be distilled having been placed in the 
boiler and a fire kindled beneath, vapor soon rises. Passing through tie 
pipe b c, it enters the worm, in which it is to be condensed. The worm is 
surrounded with cold water, with which the vat is filled, and the vapor is 
soon cooled down into a liquid form, and issues from the lower extremity of 



mation of vapor. Which makes the hotter fire, green wood or dry, — and why? 
533 What is meant by the Condensation of vapor? .539. What is Distillation ? On 
what fact is the process based ? To whom was distillation early known ? What to 
an apparatus for distilling called ? 540. With the aid of Fig. 215, describe the stilly 



INCANDESCENCE. 213 

the worm, falling into a vessel prepared to receive it. To condense the va- 
por, the water in the vat must be kept cold. For this purpose, a stream is 
kept flowing into it through the pipe p_p, while a similar stream of water 
partially warmed by the hot vapor as constantly escapes at q. By this pro- 
cess water may be obtained perfectly pure, as the earthy matter dissolved in 
it is not converted into vapor, but remains behind in the boiler. "With a 
similar apparatus, spirituous liquors are distilled from grain. 

541. Incandescence. — When a body is raised to a cer- 
tain very high temperature, it begins to emit light as well 
as heat. This state is called Incandescence, or Glowing 
Heat. 

An incandescent body becomes successively dull red, 
bright red, yellow, and white. All solids and liquids, not 
previously converted into vapor by heat, become incan- 
descent. The temperature at which incandescence com- 
mences is the same for all bodies, and may be set down at 
977 degrees of Fahrenheit's Thermometer (see § 544). 

Instrument* for measuring Heat. 

542. The expansion of bodies by heat furnishes us the 
means of measuring changes of temperature. Liquids, 
which are easily affected, are used for measuring variations 
in moderate temperatures. Solids, which require a higher 
degree of heat to expand them perceptibly, are used for 
measuring variations in elevated temperatures. Hence we 
have two instruments, the Thermometer and the Pyrom- 
eter. 

543. The Thermometer. — The Thermometer is an in- 
strument in which a liquid, usually mercury, is employed 
for measuring variations that occur in moderate tempera- 
tures. 

The thermometer (see Fig. 216) consists of a tube closed at one end and 
terminating in a bulb at the other. The bulb and part of the tube contain 
mercury, above which is a vacuum, all air having been excluded before the 
top of the tube was closed. Expanded by heat, the mercury rises in the 

md its mode of operation. 541. What is Incandescence ? What colors mark the 
successive stages of incandescence ? What substances become incandescent ? At 
what temperature does incandescence commence? 542. What means have we ol 
measuring changes of temperature ? In what cases are liquids used ? In what, sol- 
Ids? Name the instruments used for measuring changes of temperature. 543. What 



214 



PYROXOMIOS. 



Fig. 216. tube ; when the temperature falls, the mercury, contracting, 
falls also. The tube is fixed in a stand or case, and has a 
graduated scale beside it for measuring the rise and fall of the 
mercury. This scale is formed in the following way : — The ther- 
mometer is brought into contact with melting ice, and the point 
at which the mercury stands is marked. It is next plunged 
in boiling water, and the point to which the mercury rises is 
also marked. The interval is then divided into a number of 
equal spaces, called degrees. 

544. As the thermometer does not indicate 
the amount of heat in a body, but merely its 
changes of temperature, the number of degrees 
into which the interval between the freezing and 
the boiling mark is divided is arbitrary. Three 
different divisions are in use : Fahrenheit's, in 
the United States, Great Britain, and Holland ; 
Reaumur's [ro'-murz\, in Spain and parts of Ger- 
many; and the Centigrade, the most convenient 
of the three, in France, Sweden, &c. 

In Fahrenheit's scale the freezing-point is called 32, the 
boiling-point, 212 ; when, therefore, the mercury stands at 0, 
or zero, it is 32 degrees below the freezing-point. In Reau- 
mur's scale the freezing-point is called 0, the boiling-point 80. 
In the Centigrade the freezing-point is 0, the boiling-point 100. 
When degrees of the thermometer are mentioned, it is usual 
the titer- to indicate the scale referred to by the letters F., R., or C, as 
mometeb. the case ma y be. Thus 40° F. means 40 degrees on Fahren- 
heit's scale ; 15° R., 15 degrees on Reaumur's scale, &c. In this country, 
when no scale is mentioned, Fahrenheit's is meant. 

545. Imperfect thermometers were in use at the beginning of the seven- 
teenth century. It is uncertain whether the honor of their invention belongs 
to Sanctorio, an Italian physician, — Drebbel, a Dutch peasant, — or Galileo. 
Various liquids have been tried ; the astronomer Roemer was the first to use 
mercury, the advantages of which are such that it has superseded all others. 

546. The Differential Thermometer. — This instrument, 



is the Thermometer? Of what does it consist ? How is the scale of the thermome- 
ter formed? 544. What is said of the number of degrees into which the scale is di- 
vided? Name the three principal scales, and tell where each is used. What are the 
freezing-point and the boiling-point respectively called in Fahrenheit's scale ? What, 
in Keaumur's scale ? In the Centigrade scale ? How are the different scales indi- 
cated ? 545. When were thermometers first used ? To whom does the honor of their 
invention belong ? What liquid has superseded all others in the thermometer ? Who 



THE DIFFERENTIAL THERMOMETER. 



215 



Fig. 217. 



represented in Fig. 217 > measures minute dif- 
ferences of temperature. 

It consists of a long glass tube, bent twice at rigbt an- 
gles, somewhat in the form of the letter U. One arm is 
furnished with a scale of 100 degrees, and each terminates 
in a bulb. The tube contains a small quantity of sulphu- 
ric acid, colored red, and so disposed that when both 
bulbs are of the same temperature it stands at on the 
scale. Let either bulb be heated ever so little more than 
the other, and the expansion of the air within will drive 
the liquid down and cause it to ascend the opposite arm to 
a distance measured by the scale. Ordinary changes of 
temperature do not affect the instrument, because both 
bulbs are acted on alike. 

547. The Pyrometer. — The Pyrometer 
(see Fig. 218) is used for measuring variations 
in elevated temperatures, and comparing the 
expansive power of different metals for a 
given degree of heat. 

Fig. 218. 



THE DIFFERENTIAL 
THERMOMETER 

A metal bar is fixed 
in an upright at one 
end by means of a 
screw, and left free to 
expand at the other. 
It there touches a pin 
projecting from a rod 
which rests against an 
opposite upright, in a 
circular support at 
each side. This rod 
terminates at one end in an arm bent at right angles, which is connected by 
a cord and pulley with an index traversing a scale marked with degrees. 
Near its extremity is a ball, the weight of which, under ordinary circum- 
stances, keeps the index at the highest point of the scale. When lamps are 
placed beneath and the bar expands, it pushes against the pin, turns the rod 





THE PYROMETER. 



first used it ? 546. For what is the Differential Thermometer employed ? Describe 
the differential thermometer, and its operation. 547. For what is the Pyrometer 



216 PYBONOMICS. 

more or less around, and thus raises the arm containing the ball and moves 
the index along the scale. The relative degree of heat applied to the bar is 
thus indicated. By keeping the heat the same, and using rods of different 
metals, we can ascertain their relative expansive power. 

Specific Heat. 

548. Put a pound of water and a pound of olive oil in 
two similar vessels, and apply heat. It will take twice as 
tong to raise the water to a given temperature as it will the 
oil. Let them cool, and the water will be twice as long in 
parting with its heat as the oil. Water, therefore, must 
receive twice as much heat as olive oil in reaching a given 
temperature. 

The relative amount of heat which a body receives in 
reaching a given temperature is called its Specific Heat, or 
its Capacity for Heat. 

549. In estimating the specific heat of bodies, that of water is taken as a 
standard. Reckoning the specific heat of water as 1, that of iron is about 
V 9 , and mercury only 733- As a general thing, the densest bodies have the 
least specific heat ; solids have less than liquids, and liquids less than gases 
and vapors. 

550. As the elastic fluids expand, they are rarefied, and their specific heat 
becomes greater. — that is, it requires more heat to raise them to a given tem- 
perature. This is one reason why the upper regions of the atmosphere are 
colder than the lower, as is found by those who ascend mountains. 

Steam. 

551. Generation op Steam. — Water is rapidly turned 
into steam at its boiling-point, which in an open vessel at 
the level of the sea is 212° F. After it commences boiling, 
water can not be raised to any higher temperature, because 
all the heat subsequently applied is absorbed by the steam 
and passes off with it. 

ased ? Describe the Pyrometer. 548. How is it proved that water must receive twic« 
as much heat as olive oil in reaching a given temperature ? What is meant by Spe- 
cific Heat ? 549. In estimating the specific heat of bodies, what is taken as a stand- 
ard ? What is the specific heat of iron ? Of mercury ? As a general thing, what 
bodies have the least specific heat ? 550. Under what circumstances is the specific- 
heat of elastic fluids increased ? What fact is thus explained ? 551. How is steam 
generated ? Why can not water, after it commences boiling, be raised to any higher 






STEAM. 



217 



If the water is in a close vessel, the steam first formed, 
being confined, presses on the water and prevents it from 
boiling as soon as before. It may now be raised to a more 
elevated temperature, for heat is not withdrawn by the 
formation of steam till it reaches a higher point. 

552. Steam has the same temperature as the water from 
which it is formed, the heat absorbed in the process of for«, 
mation becoming latent, — that is, not appearing. When 
it is generated from water in an open vessel, its temper- 
ature is 212° ; in a confined vessel it will be higher, ac- 
cording to the pressure on the surface of the water. 

553. Steam is colorless and invisible. When cooled by 
contact with the atmosphere, it begins to turn back into a 
liquid state, and assumes a grey mist-like appearance. Look 
at the spout of a tea-kettle full of boiling water. For half 
an inch from the extremity nothing can be seen ; beyond 
that, the steam, cooling and beginning to 
condense, becomes visible. 

554. The generation and properties of steam may- 
be understood from Fig. 219. AB represents the in- 
side of a tall glass tube, the section of which has an 
area of one square inch. The tube is closed at its 
lower end, and contains a cubic inch of water, D, and 
resting on it a tightly-fitting piston, C. A cord, fast- 
ened to the piston, is carried round the wheel E, and 
attached to the weight F. F is made just heavy enough 
to counterbalance the piston and its friction against 
the tube. Suppose a thermometer to be placed in 
the water, and apply heat at the bottom of the tube. 
As soon as the thermometer indicates a temperature 
of 212°, tbe piston begins to rise, leaving a space ap- 
parently empty between it and the water. The fire 
continues to impart heat to the water, but the mer- 
cury in the thermometer remains stationary at 212° ; 
the piston keeps rising, and the water begins to di- 
minish. If the process were continued and the tube 
were long enough, the piston would at last reach a 



Fi<?. 219. 




temperature ? Under what circumstances may water be raised to a higher tempera- 
ture than 212° ? 552. What is the temperature of steam ? 553. What is the color of 
6team ? Explain the mist-like appearance a short distance from the spont of a boiling 
tea-kettle. 554. With the aid of Fig. 219, show the process of generating steam, and 



218 PYRONOMICS. 

height of nearly 1,700 inches, by which time the water would entirely disap- 
pear. If the tube were then weighed, though nothing could be seen in it but 
the piston, it would be found to have exactly the same weight as at first. 
The water would simply be converted into steam, and thus increased in vol- 
ume 1,700 times. The piston, with the pressure of the atmosphere on it 
(which is 15 pounds, the area of the piston being one square inch), would be 
raised 1,700 inches. 

All the time steam is forming, a uniform amount of heat is applied to the 
tube. As the mercury in the thermometer rises no higher than 212°, it is 
evident that the heat imparted after it reaches that point is absorbed by the 
steam and becomes latent. To determine the amount of this latent heat, we 
must compare the time required to raise the water from the freezing to the 
boiling point with the time that elapses from the commencement of boiling 
till the water disappears. We shall find that the latter interval is 5% times 
as great as the former ; and, since from the freezing-point (32°) to the boiling- 
point (212°) is 180°, we conclude that the amount of heat absorbed is 5Va 
times 180°, or nearly 1,000 degrees. That is, the heat applied would have 
raised the water to a temperature of nearly 1,000°, if it could have remained 
in the liquid state. 

555. If, besides the pressure of the atmosphere on P, a weight of 15 pounds 
were placed on it, it would be said to have a pressure of two atmospheres. 
Steam, in this case, would not commence forming till the water reached a 
temperature of 251^2 degrees ; and, when the whole was evaporated, the pis- 
ton would stand only about half as high as before. Under a pressure of three 
atmospheres, the piston would be raised about one-third as high, &c. ; the 
mechanical force developed in the evaporation of a given quantity of water 
remaining nearly the same. This force, for a cubic inch of water, is suffi- 
cient to raise a ton a foot high. 

556. Steam has a high degree of elasticity and expansi- 
bility. Under a pressure of two atmospheres, or 30 pounds 
to the square inch, it would raise the piston in the above 
experiment about 850 inches; if 15 pounds were removed 
from the piston, the expansive force of the steam would 
drive it up 850 inches farther. 

557. Condensation of Steam. — Steam retains its form 
only as long as it retains the latent heat absorbed. The 



describe some of its properties. When water is converted into steam, how many 
times is its volume increased? How is this proved with the apparatus just de- 
scribed ? Prove that heat becomes latent in the steam. How can the amount of 
latent heat be determined ? 555. When is steam said to have a pressure of two at- 
mospheres? How high would the piston then be raised ? How high would the piston 
be raised under a pressure of three atmospheres ? How great is the mechanical force 
developed in evaporating a cubic inch of water ? 556. Prove the expansibility of 
?team. 557. How long does steam retain its form ? When is it condensed ? Show 



THE STEAM-ENGINE. 219 

moment it is forced to part with this heat, it is turned back 
into the liquid form, or condensed. 

In the above experiment, after the piston has been raised 1,700 inches, let 
the fire be removed, and cold water be applied to the surface of the tube. 
The latent heat will be abstracted, and the steam will be condensed and form 
once more a cubic inch of water at the bottom of the tube. As the steam 
condenses, successive vacuums are produced ; and the piston, forced down 
by the pressure of the atmosphere, descends, and finally rests on the water 
as at first. 

By applying heat again, the process may be repeated. An up-and-down 
motion may in this way be communicated to the piston ; and the piston may 
be connected with machinery, which will thus be set in motion by the al- 
ternate evaporation of water and condensation of steam. This was the prin- 
ciple of the Atmospheric Engine, which was once extensively used, but has 
now been superseded. 

Tbe Steam-Engine. 

558. Hero's Engine. — Steam and some of its proper- 
ties appear to have been known to the ancients centuries 
before the Christian era. Hero, of Alexandria, who flour- 
ished about 200 years b. c, has left us a description of a 
steam-engine by which machinery could be set in motion. 

Fig. 220 represents Hero's Fig. 220. 

engine. A hollow metallic 
globe is supported by pivots, 
and provided with a number 
of jets equally distant from 
the pivots, and bent at right 
angles near their outer end. 
As soon as steam is introduced 
into the globe, it issues vio- 
lently from the mouth of each 
jet, while on the opposite side 
of each it presses without be- 
ing able to escape. This un- 
balanced pressure makes the 
globe revolve. Machinery may hbko's steam-engine. 

be set in motion by means of a band connected with this apparatus. 

559. Hero's was a simple rotatory engine. No use was made of it for 




how it may be condensed in the above experiment. What follows the condensation 
of the steam ? How may an up-and-down motion be communicated to the piston ? 
What engine was constructed on this principle ? 558. How long ago was steam 
known ? Who has left us a description of a steam-engine ? Describe Hero's engine. 



220 PYRONOMICS. 

2,000 years ; but the principle involved has been revived, and is applied in 
rotatory engines at the present day. 

560. De Garay's Engine. — In 1543, a Spaniard, by the 
name of De Garay, undertook to propel a vessel of 200 tons 
in the harbor of Barcelona by the force of steam. He kept 
his machinery a secret, but it was observed that a boiler 
and two wheels constituted the principal part of his appa- 
ratus. The experiment succeeded. The vessel moved 
three miles an hour, and was turned or stopped at pleasure; 
but the Emperor Charles V., by whose order the trial was 
made, never followed the matter up, and De Garay and his 
invention were forgotten. 

561. Engines of De Caus and Branca. — In 1615, De 
Caus, a French mathematician, devised an apparatus by 
which water could be raised in a tube through the agency 
of steam. A few years afterwards, an Italian physician, 
named Branca, ground his drugs by means of a wheel set 
in motion by steam. The steam was led from a close ves- 
sel, in which it was prepared, and discharged against flanges 
on the rim of the wheel. 

562. The Marquis of Worcester's Engine. — The Mar- 
quis of Worcester, by many regarded as the inventor of the 
steam-engine, greatly improved on the imperfect attempts 
of those who had preceded him. 

Some say that Worcester derived his ideas from De Caus. Others claim 
that his invention was purely original, and the result of reflections to which 
he was led during his imprisonment in the Tower of London, in 1656, for 
plotting against the government of Cromwell. Observing how the steam kept 
moving the lid of the pot in which he was cooking his dinner, he could not 
help thinking that this power could be turned to a variety of useful purposes, 
and set about devising an engine in which it might be applied to the raising 
of water. 

The Marquis of Worcester generated his steam in a boiler, and led it by 
pipes to two vessels communicating on one side with the reservoir from 
which it was to be drawn, and on the other with the cistern into which it 
was to be discharged. 



559. What sort of an engine was Hero's, and what is said of it? 560. Give an account 
of De Garay's engine, and the experiment made with it. 561. Give an account of De 
Caus's engine. Of Branca's. 562. Whom do many regard as the inventor of the steam- 
engine ? What claim has he to the honor ? How was he led to reflect on the subject f 



THE STEAM-ENGINE. 



'221 



563. Papin's Engine. — The next step was taken by Pa- 
pin, who devised the mode of giving a piston an up-and- 
down motion in a cylinder by alternately generating and 
condensing steam below a piston. 

564. Saveey's Engine. — Captain Thomas Savery, in 
1698, constructed an engine superior to any before invent- 
ed. He was led to investigate the subject by the following 
occurrence. Having finished a flask of wine at a tavern, he 
flung it on the fire, and called for a basin of water to wash 
his hands. Some of the wine remained in the flask, and 
steam soon began to issue from" it. Observing this, Savery 
thought that he would try the effect of inverting the flask 
and plunging its mouth into the basin of cold water. No. 
sooner had he done this than the steam condensed, and the 
water rushing into the flask nearly filled it. Confident that 
he could advantageously apply this principle in machinery, 
Savery rested not till he invented an engine which was em- 
ployed with success in drawing off the water from mines. 

565. The principle on which Savery's engine 
worked, may be understood from Fig. 221. S is a 
pipe connecting a boiler in which steam is genera- 
ted (and which does not appear in the Figure) with 
a cylindrical vessel, C, called thereceiver. I is known 
as the injection-pipe, and is used for throwing cold 
water into the receiver to condense the steam. The 
steam-pipe, S, and the injection-pipe, I, contain the 
stop-cocks, G, B, which are moved by the common 
handle, A, so arranged that when one is opened the 
other is closed. F is a pipe which descends to the 
reservoir whence the water is to be drawn, and is 
commanded by the valve V, opening upward. ED 
is a pipe leading from the bottom of the receiver up 
to the cistern, into which the water is to be discharged, 
the valve Q, opening upward. 

Operation. — To work the engine, open the stop-cock G, which of course 
involves the shutting of B. The steam rushes in through S, and fills the re- 
ceiver C, driving out the air through the valve Q. When C is full, shut G 




This pipe contains 



How was the Marquis of Worcester's apparatus arranged ? 563. Who took the next 
step ? What was Papin's improvement ? 564. Who constructed a superior engine in 
1698? Eelate the circumstances that led Savery to investigate the subject. 565. With 
the aid of Fig. 221, describe the parts of Savery's engine. Explain its operation. 



222 PYR0N0MICS. 

and open B. Cold water at once enters through the injection-pipe and con 
denses the steam in C. A vacuum is thus formed, and the water in the res- 
ervoir or mine, under the pressure of the atmosphere, forces open the valve 
V, and rushes up through F into G, till the receiver is nearly filled. G is then 
opened and B closed ; when the steam again enters through S, and by its 
expansive force opens the valve Q, and drives the water up through E D into 
the cistern. 

566. Newcomen's Engine. — Savery'6 engine was em- 
ployed only for raising water ; but Newcomen, an intelli- 
gent blacksmith, extended its sphere of usefulness, by con- 
necting a piston, worked up and down on Papin's principle, 
with a beam turning on a pivot, by means of which ma- 
chinery of different kinds could be set in motion. 

567. About this time, also, the engine was made self-acting through the 
ingenuity of Humphrey Potter, a lad employed to turn the stop-cocks. Pre- 
ferring play to this monotonous labor, he contrived to fasten cords from the 
beam to the handle of the stop-cocks, in such a way that the latter were 
opened and closed at the proper times, while he was away, enjoying himself 
with his companions. His device was after a time found out, and saved so 
much labor that it was at once adopted as an essential part of the machine. 

568. Watt's Engine. — The genius of James Watt 
brought the steam-engine to such perfection that but little 
improvement has since been made in it. Gifted with re- 
markable mathematical powers and a reflective mind, he 
commenced his experiments in 1763. Having been em- 
ployed to repair one of Newcomen's engines, he soon per- 
ceived that there was a great loss in consequence of having 
every time to cool down the receiver from a high degree 
of heat before the steam could be condensed. This diffi- 
culty he remedied by providing a separate chamber called 
a condenser, to which the steam was conveyed and in which 
it was condensed. He also made the movement of the pis- 
ton more prompt and effective by introducing steam into the 
cylinder alternately above and below it. The Double- 
acting Condensing Steam-engine, as improved by Watt, and 



666. What was the only purpose for which Savery's engine was employed ? Who ex- 
tended its usefulness, and how ? 567. Give an account of Humphrey Potter's im- 
provement, and the circumstances under which it was devised. 568. Who brought 
the steam-engine to comparative perfection ? When did Watt commence his exper- 
iments? What disadvantage did he perceive that Newcomen's engines labored un- 
dar? flow did he remedy the difficulty ? What other improvement did he make? 



THE STEAM-ENGINE. 



223 



now generally constructed for manufacturing establishments, 
is represented in Fig. 222. 

569. Description of the Parts. — A is the cylinder, in which the piston T 
works. This piston is connected by the piston-rod R with the working-beam 

Fig. 222. 

"W 






THE DOUBLE-ACTING CONDENSING STEAM-ENGINE. 

V W, which turns on a pivot, U. The other end of the working-beam, (\ 
imparts a rotary motion to the heavy fly-wheel X Y, by means of the connect* 
ing-rod P and the crank Q. The fly, as explained on page 125, regulates the 
motion, and is directly connected with the machinery to be moved. Steam 



669. Describe the parts of Watt's Double-acting Condensing Engine. Show how th« 



224 



PYRONOMICS. 



is conveyed to the cylinder A from the boiler (which is not seen in the fig- 
ure), through the steam-pipe B, which is commanded by the throttle-valve C. 
This valve is connected with the governor D, in such a way as to be opened 
when the supply of steam is too small and closed when it is too great. 

Communicating with the cylinder at its top and bottom on the left, are 
two hollow steam-boxes, E, E, each of which is divided into three compartments 
by two valves. F is called the upper induction-valve, and opens or closes 
communication between the steam-pipe and the upper part of the cylinder, 
so as to admit or intercept a supply of steam. G, called the upper exhaustion- 
valve, opens or closes communication between the upper part of the piston 
and the condenser K, so that the steam may either be allowed to escape into 
the latter or confined in the cylinder. The lower induction-valve g, and the 
lower exhaustion-valve f, stand in the same relation to the lower part of the 
cylinder, the former connecting it with the steam-pipe, and the latter with 
the condenser K. These valves are connected by a system of levers with a 
common handle, H, called a spanner, which is made to work at the proper in- 
tervals by a pin projecting from the rod L, which is moved by the working- 
beam. The spanner works so as to open and close the valves by pairs. When 
it is pressed up, it opens F and/, and closes G and g ; when pressed down, 
it closes F and/ and opens G and g. 

Below is the condensing apparatus, consisting of two cylinders, I and J, 
immersed in a cistern of cold water. A pipe, K, having an end like the rose 
of a watering-pot, conveys water from the cistern to the cylinder I (the sup- 
ply being regulated by a stop-cock), and thus condenses the steam which is 
from time to time admitted into I. The other cylinder, J, called the air-pump, 
contains a piston with a valve in it opening upward, which works like the 
bucket of a common pump, and draws off the surplus water that collects at 
the bottom of the cylinder I into the upper reservoir S. The hot-water pump 
M then conveys this water to the cistern that supplies the boiler. To keep 
the water around the condensing apparatus at the right temperature, a fresh 
supply is constantly introduced through the cold-water pump N ; which, like 
the hot-water pump and the air-pump, is kept in operation by rods connected 
with the working-beam. 

570. Operation. — The working of the engine is as follows : — Let the piston 
be at the top of the cylinder, and all the space below be filled with steam. 
The upper induction-valve and the lower exhaustion-valve are then opened 
by the spanner, while the upper exhaustion-valve and the lower induction- 
valve are closed. By this means steam is introduced above the piston, while 
the steam beneath is drawn off into the condenser, where it is converted into 
water. The pressure of the steam above at once forces the piston to the bot- 
tom of the cylinder. Just at this moment the spanner is moved in the oppo- 
iite direction, and the valves that were before opened are closed, while those 
that were previously closed are opened. The steam is now admitted beneath 
the piston, and the steam above is drawn off into the condenser and convert- 
ed into water as before. While this action is going on, the cold-water pump 

valves work. Describe the condensing apparatus. 570. How is the engine worked? 



THE STEAM-ENGINE. 



225 



is constantly supplying the cistern in which the condenser is immersed ; while 
the air-pump is drawing off the hot water from the condenser to the upper 
reservoir, whence it is conveyed by the hot-water pump to the cistern that 
supplies the boiler. An up-and-down motion is thus communicated to the 
piston, and by it to the working-beam, which causes the fly to revolve, and 
moves the machinery with which it is connected. 

571. The Governor. — The Governor, an ingenious piece 
of mechanism, by which the throttle-valve in the steam- 
pipe is opened and closed, and the supply of steam regu- 
lated as the machinery requires, is worthy of further de- 
scription. 

The governor and its Fig. 223. 

connection with the throt- 
tle-valve are represented in 
Fig. 223. It consists of two 
heavy balls of iron, E, E, 
suspended by metallic arms 
from the point e. At ethey 
cross, forming a joint, and 
are continued to/,/, where 
they are attached by pivots 
to other bars,/ h,f h. These 
bars are joined to one end 
of a lever, the other end of 
which, H, is connected at 
W with the handle of the 
valve Z. The spindle DD, to which the balls are attached, turns with the 
fly-wheel. When the fly-wheel revolves very rapidly , the balls E E, under 
the influence of the centrifugal force, fly out from the spindle, and with the 
aid of the barsfh,fh, pull down the end of the lever g. The other end, H, 
is of course raised, and with it the handle of the valve Z, which is thus made 
to close the mouth of the steam-pipe A and cut off the supply of steam. On 
the other hand, when the motion of the fly diminishes, the centrifugal force 
of the balls E E also diminishes, and they fall towards the spindle. The near- 
er end of the lever g is thus raised, while the end H is depressed. The valve 
Z is by this means opened, and admits a full supply of steam. The governor 
thus acts almost with human intelligence, now admitting, and now cutting 
off the steam, just as is required. 

572. The Boiler. — The boiler is made of thick wrought- 
lron or copper plates, riveted as strongly as possible, so as 
to resist the expansive force of the steam generated within. 




THE GOVERNOR. 



flow »re the cisterns supplied? 571. What is the Governor? Describe the gov- 
ernor, and its connection with the throttle-valve. Show the workings of the gov 
15 



226 PYRONOMICS. 

The fire is applied in an apartment beneath or within *he 
boiler called the Furnace. 

Boilers are made of different shapes, but are generally 
cylindrical, because this form is one of the strongest. Watt 
made his concave on the bottom, in order to bring a greater 
extent of surface in contact with the flame. 

573. The Safety Valve. — The pressure on the boiler, m 
consequence of the expansive force of steam, is immense. 
If it is allowed to become too great, the boiler bursts, often 
with fatal effects. To prevent such catastrophes, a Safety 
Valve is fixed in the upper part of the boiler, which is forced 
open and allows some of the steam to escape whenever the 
pressure exceeds a certain amount. A lever, with a weight 
which slides to and fro on its arm, is attached to the valve ; 
and the engineer, by placing the weight at different dis- 
tances, can determine the amount of pressure which the 
boiler shall sustain before the valve will open. 

574. Kinds of Engines. — Engines are divided into two 
kinds, Low Pressure and High Pressure. 

In the Low Pressure Engine, one form of which has been 
described above, the steam is carried off and condensed ; 
while in the High Pressure Engine it is allowed to escape 
into a chimney, and thence into the open air. The latter, 
having no condensing apparatus, is much the simpler in its 
construction. It is noisy when in operation, in consequence 
of the puffing sound made by the steam as it escapes. 

575. As regards their use, engines may be divided into 
three classes ; Stationary Engines, employed in manufactur- 
ing, Marine Engines, for propelling boats, and Locomotive 
Engines, for drawing wheeled carriages. 

576. The Locomotive Engine. — The Locomotive is a 
high pressure engine. The principle on which it works may 
be understood from Fig. 224. 



ernor. 572. Of what is the boiler made ? Where is the fire applied? What is tho 
usual shape of boilers ? What shape did Watt make his, and why ? 573. What is the 
use of the Safety Valve ? How is it worked ? 574. How are engines divided ? What 
constitutes the difference between Low Pressure and High Pressure Engines ? Which 
are the simpler ? Which are the more noisy, and why? 575. As regards their use, 



THE LOCOMOTIVE ENGINE. 

Fig. 224. 



227 




The cylinder A in this engine is horizontal instead of vertical, and the pis- 
ton works horizontally. B, the piston-rod, is connected by a crank, D, with 
the axle E E of the wheels, F, F. The piston, moving alternately in and out 
of the cylinder, with the aid of the crank causes the axle and wheels to re- 
volve ; and the wheels, by their friction on the rails, move forward the en- 
gine and whatever may be attached to it. The heavy line represents the 
position of the parts when the piston is at the remote extremity of the cylin- 
der ; the dotted line shows their position, when the piston has reached the 
other end. Steam is first introduced on one side of the piston, and then on 
the other, being allowed to escape as soon as it has done its work, — that is, 
driven the piston to the opposite extremity. The rest of the machinery con- 
sists of arrangements for boiling the water, for regulating the admission of 
steam into the cylinder and its discharge, for providing draught for the fire, 
and for giving the driver the means of starting and stopping the engine, and 
reversing the direction of its motion. 

577. History. — Watt seems to have been the first to 
conceive the idea of propelling wheeled carriages by steam ; 
but he was so engaged in perfecting the stationary engine 
that he did not attempt to carry out his idea. William 
Murdoch, in 1784, first constructed a locomotive. Though 
little more than a toy, it worked successfully, and travelled 
so fast that on one occasion it's inventor in vain tried to 
keep pace with it. 

Eighteen years passed before any use was made of Mur- 
doch's invention ; at the end of that time, in 1802, Richard 
Trevithick publicly exhibited a locomotive engine, so con- 



into what three classes may engines he divided ? 576. With Fig. 224, show the prin- 
ciple on which the locomotive engine works. What does the rest of the machinery 
consist of? 577. Who first conceived the idea of the locomotive engine? Who first 
carried out the idea ? What is said of Murdoch's engine ? Who exhibited an im- 






228 EXAMPLES FOR PRACTICE. 

structed that it could be used for transporting cars. Im- 
portant modifications and improvements have since been 
made, for many of which the world is indebted to George 
Stephenson, who shares with Trevithick the honor of this 
great invention. 

EXAMPLES FOR PRACTICE. 

1. {See § 510.) A joint of meat stands 2 feet from a fire, a fowl 4 feet; how 

does the heat which strikes the former compare with that received by 
the latter ? 

2. How does the heat which my finger receives from the flame of a candle, 

when held at the distance of an inch, compare with what it receives 
when held a foot from the flame ? 

3. If we were but one-fifth of our present distance from the sun, how many 

times as much heat would we receive from it ? 

4. The planet Neptune is about 30 times as far from the sun as the earth is ; 

how does its solar heat compare with ours ? 

5. To receive a certain amount of heat from a fire, an object is placed 3 feet 

from it ; to receive only one-fourth as much heat, how far from the fire 
must it be placed ? 

6. (See §526.) A quantity of water at the freezing-point measures 22 gallons ; 

how much will it measure when its temperature has increased to the 
boiling-point ? 

7. I have a vessel which holds 46 gallons ; how much water at a temperature 

of 32° must I put in it, to exactly fill the vessel when it boils ? 

8. What will be the increase in measure of 18 gallons of alcohol, when raised 

from 32° to 212° ? What will be the increase in weight ? 

9. (See § 554.) Under a pressure of one atmosphere, how many cubic inches 

of steam will be generated from 2 cubic inches of water? From 10 cubic 
inches of water ? 

10. If 3,400 cubic feet of steam (under a pressure of one atmosphere) be con- 
densed, how much water will it make ? 

11. (See § 555.) Under a pressure of two atmospheres, about how many cubic 
inches of steam will two inches of water generate ? How many, under 
a pressure of three atmospheres ? 

12. About how many cubic inches of steam will be required, to raise 10 tons 
10 feet high ? If the steam were condensed, how many cubic inches of 
water would it make ? 



proved locomotive in 1802 ? Who subsequently made important improvements in 
the locomotive? 



optics. 229 

CHAPTER XIV. 

OPTICS. 

578. Optics is the science that treats of light and vision. 

Nature of Light. 

579. Light, as stated in § 471, is one of the modes of 
force originating in molecular motion, by the action of 
which upon the eye we are enabled to see. 

580. Undulatory Theory. — All space is believed to be 
pervaded by an exceedingly subtile and almost infinitely 
elastic fluid, to which the name ether is applied. The vi- 
brating atoms in a luminous body millions of miles away, 
communicating their motion to the contiguous ether, cause 
it to move in minute waves, like the surface of a pond 
rippled by throwing in a stone. These undulations are 
transmitted with inconceivable rapidity to the eye, strike 
the sensitive membrane that lines it, and produce the sen- 
sation of light. 

581. This Undulatory Theory, as it is called, advanced 
by Descartes \da-kart'\ but first definitely laid down by 
Huyghens, explains most of the phenomena of optics, and 
is now universally received. 

582. Light was formerly thought to consist of minute particles of mat- 
ter thrown off from luminous hodies, which struck the eye and produced 
the sensation of light, just as particles thrown off by an odoriferous sub- 
stance affect the organ of smell. This was known as the Corpuscular, or 
Emission Theory. It was held as long ago as the days of Pythagoras, and 
was received by Newton ; but, failing to account for many of the facts more 
recently discovered, it has been wholly superseded by the Undulatory 
Theory, set forth above. 

5T8. What is Optics? 5T9. What is Lisrht? 580. According: to the Undulatory 
Theory, hy what is space pervaded, and how is lisrht produced ? 5S1. By -whom was 
the Undulatory Theory maintair >d ? 582 State the chief points of the Corpuscular 
Theory. By whom was it held ? Which of these theories is now universally received ? 



230 



OPTICS. 




583. ifoys.— Rays are single lines of light, the smallest 
distinct parts into which light can be resolved. 

Fig. 225. Fig. 226. Fig. 227. Rays of light from the 

same body either move in 
parallel lines, as in Fig. 
225 ; or diverge, that is, sep- 
arate from each other, as in 
Fig. 226 ; or converge, that 
is, come together at a point called the Focus, as in Fig. 227. 

A Beam of light is a collection of parallel rays. 

A Pencil of light is a collection of rays not parallel. 

A Diverging Pencil is a collection of diverging rays. 

A Converging Pencil is a collection of converging rays. 

Division of Bodies. 

584. Self-luminous and Non-luminous Bodies. — As 
regards the production of light, bodies are divided into two 
classes, Self-luminous and Non-luminous. 

Self-luminous bodies are those which are seen by the 
light that they themselves produce ; as, the sun, the stars, 
a lighted candle. 

Non-luminous bodies are those that produce no light of 
their own, but are seen only by that of other bodies. The 
moon is non-luminous, its light being borrowed from the 
sun. The furniture in a dark room is non-luminous, being 
invisible until the light of the sun, a lamp, or some other 
luminous body, is admitted. 

Many non-luminous bodies, when exposed to a heat of 977° F., become 
incandescent, and grow brighter and brighter with every increase of temper- 
ature beyond that point, till they reach a white heat. This is a striking proof 
of the connection between light and heat. 

585. Transpabent, Translucent, and Opaque Bodies. 



583. What are Bays ? How may rays move ? What is a Beam of light ? What is a Pen- 
cil of light? What is a Diverging Pencil ? What is a Converging Pencil? 5S4. As 
regards the production of light, how are hodies divided ? What are Self-luminous 
bodies? What are Non-luminous bodies? Give examples. What striking proof 
have we of the connection between light and heat ? 585. As regards the transmission 



TRANSPARENT AND OPAQUE BODIES. 231 

*— As regards the transmission of light, bodies are divided 
into three classes ; Transparent, Translucent, and Opaque. 

Transparent bodies are such as allow light to pass freely 
through them ; air, water, glass, are transparent. 

Translucent bodies are such as allow light to pass through 
them, but not freely ; ground glass, thin horn, paper, are 
translucent. 

Opaque bodies are such as do not allow light to pass 
through them ; wood, stone, the metals, are opaque. 

Transparent and opaque are relative terms. No substance transmits 
light without intercepting some by the way. It is computed that the sun's 
rays lose nearly one-fourth of their brilliancy by passing through the earth's 
atmosphere ; and that, if this atmosphere extended fifteen times as far from 
the surface as it now does, we should receive no light at all from the sun, 
but should be plunged in perpetual night. On the other hand, an opaque 
substance, if made very thin, may become transparent. Gold leaf, for in- 
stance, held in the sun's rays, transmits a dull greenish light. 

586. Media. — By a Medium (plural, media) is meant 
any substance through which a body or agent moves in 
passing from one point to another. Air is the medium in 
which birds fly ; water, the medium in which fish swim ; 
ether, the medium in which the planets move. In connec- 
tion with light, any substance through which it passes is a 
medium ; as air, water, glass, &c. 

587. A Uniform Medium is one that is of the same 
composition and density throughout. 

Sources of Light. 

588. The principal sources of light are nearly the same 
as those of heat ; viz., the Sun and Stars, Chemical Action, 
Mechanical Action, Electricity, and Phosphorescence. 

Most of our artificial light is produced by chemical action, as exhibited in 
the process of combustion (see § 479). To this is due the light of lamps, can- 

of light, how are bodies divided ? What are Transparent bodies ? What are Trans- 
lucent bodies ? What are Opaque bodies ? What is said of the terms transparent 
and opaque f How much of their brilliancy do the sun's rays lose in passing through 
the atmosphere ? What would be the consequence if the atmosphere extended fif- 
teen times as far as at present ? How may an opaque substance be made transparent? 
586. What is a Medium? Give examples. 587. What is a Uniform Medium? 
688. Name the principal sources of light. How is most of our artificial light pro- 



232 optics. 

dies, gas, ftres, etc. — The mechanical action involved in percussion is also 
a source of light. Sparks are produced when flint and steel are struck vio- 
lently together. — Lightning, and the sparks given off from the electrical 
machine, are examples of light produced by electricity. — Phosphorescent 
light is unaccompanied with perceptible heat. It is seen in decayed wood, 
fire-flies, glow-worms, and certain marine animals. Vast tracts of ocean 
are sometimes rendered luminous by myriads of phosphorescent creatures. 

589. The Sun and Stars, sources of light. — The sun 
Las already been mentioned (§ 474) as the great natural 
source of heat and light to the earth. Notwithstanding 
the loss of some of its brightness in consequence of passing 
through our atmosphere, its light is more intense than any 
other with which we are acquainted. The most dazzling 
artificial lights look like black specks, when held up be- 
tween the eye and the sun, so much more brilliant is the 
latter. It would require the concentrated brightness of 
5,563 wax candles at the distance of a foot, to equal the 
light which we receive from the sun at a distance of 
91,430,000 miles. 

The fixed stars are the suns of other systems. Like our 
sun, they are self-luminous, and therefore sources of light, 
though unimportant to us as such by reason of their great 
distance. The light we get from Sirius, one of the bright- 
est of the fixed stars, is only one twenty-thousand-millionth 
of what we receive from the sun. When the sun shines, 
the stars are invisible, their light being lost in his superior 
brightness. 

The light of some of the stars is so faint, that it is entirely absorbed by 
the atmosphere before it reaches the eye of an observer at the level of the sea. 
This is the reason why more stars are visible from the top of a mountain than 
from its base. 

590. The moon and planets are non-luminous, receiving from the sun the 



duced ? Give an example of light produced by mechanical action. Of light pro- 
duced by electricity. What is the peculiarity of phosphorescent light ? In what is 
it seen ? 589. What is the great natural source of light to the earth ? How does the 
sun's light compare with other lights with which we are acquainted ? Prove this. 
To how many wax candles is the light received from the sun equal? What are th( 
fixed stars 9 What renders them unimportant to us, as sources of light? How does 
the light of Sirius compare with that of the sun? Why are the stars invisible in the 
day-time? Why can more stars be seen from the top of a mountain than from its 
base ? 590. What heavenly bodies are non-luminous ? What follows with r.^peet to 



PROPAGATION OF LIGHT. 233 

light with which they shine. This light, reflected to the earth, is much in- 
ferior in brigntness to that received directly from the sun. The latter body, 
for example, gives us 547,500 times as much light as the full moon. 

Propagation of Light. 

591. Direction. — Light radiates from every point of a, 
luminous surface in every direction. 

The flame of a candle can be seen by thousands of persons at once, be 
■jause a ray from the flame meets the eye of each. Within the immense space 
belonging to the solar system, there is no point at which an observer can be 
placed without seeing the sun, provided no opaque body intervenes. From 
the sun, therefore, and from every luminous body, an infinite number of rays 
proceed. 

592. In a uniform medium, light is propagated in 
straight lines. 

Look through a straight tube at the sun, and you see it ; not so, if you 
look through a bent or curved tube. Place a book between your eye and a 
gas-burner ; the latter is not visible, because, to reachyour eye, the light from 
it would have to deviate from a straight line. Darken a room, and admit a 
sunbeam through a small hole in a shutter. Its path, marked out by the 
floating dust that it illuminates, is seen to be a straight line. 

593. The rays proceeding in straight lines from different particles of a 
luminous body cross at every point within the sphere of its illumination, but 
without at all interfering with each other ; just as different forces may act 
on an object, and each produce the same effect as if it acted alone. A dozen 
candles will shine through a hole in the wall of a dark room, and each with 
the same intensity and direction as if no other rays than its own traversed 
the narrow passage. 

594. Velocity. — Light travels with the enormous ve- 
locity of 185,000 miles in a second. While you count one, 
it goes eight times round the earth ; it would take the 
swiftest bird three weeks to fly once around it. Light 
traverses the space between the sun and the earth in about 
8£ minutes ; a cannon-ball would be seventeen years in 
going the same distance. 



a 



their light ? How does the moon's light compare with the sun's ? 591. What is the 
law for the direction of radiated light ? Show the truth of this law in the case of a 
candle and the sun. 592. In a uniform medhim, how is light propagated ? Prove 
this by some familiar experiments. 593. What is said of the rays proceeding in 
straight lines from different particles of a luminous body ? Illustrate this with can- 
dles shining through a hole. 594 What is the velocity of light? How does it com- 
pare with that of the swiftest bird ? With that of a cannon-hall ? By whom was the 



234 optics. 

The velocity of light was discovered accidentally, by Roemer, an eminent 
Danish astronomer, when engaged in a series of observations on one of ihe 
moons of the planet Jupiter. This moon, in a certain part of its path, be- 
comes invisible to an observer on the earth, in consequence of getting be- 
hind its planet. Knowing that the revolutions of the moon must be per- 
formed in the same time, Roemer supposed that the intervals between these 
invisible periods would of course be uniform. To his surprise, he found that 
they differed a little every time ; increasing for six months (at the expiration 
of which, the eclipse was 16 min. 26 sec. later than at first), and then de- 
creasing at the same rate for a similar period, till at the end of the year he 
found the interval precisely the same as at first. The conclusion was inevi- 
table. The discrepancy was caused by the difference in the earth's distance. 
If the first observation was made when the earth was at that point of her 
orbit which was nearest to Jupiter, six months afterwards she would be at 
the most distant point ; and the light from Jupiter's moon, to reach the 
observer's eye, would have to travel the whole distance across the orbit 
(about 183,000,000 miles) farther than before. Here was the key to a grand 
discovery. If light is 16 min. 26 sec, or 986 seconds, in travelling 183,- 
000,000 miles, it is easy to find how far it travels in one second. 

595. Intensity at different Distances. — The inten- 
sity of light, like that of radiant heat, diminishes as the 
square of the distance increases. 

Let several objects be placed respectively 1 foot, 2 feet, 3 feet, &c. from a 
luminous body; they will then receive different degrees of light proportioned 
to each other as 1, 1 / i , 1 / 9 , &c. — A planet twice as far from the sun as the 
earth is, would receive from it only J / 4 as much light ; one three times as far, 
y 9 as much ; one ten times as far, i j 100 as much. 

596. This is illustrated with Fig. 228. A 
square card placed at A, a distance of 1 foot 
from the candle, receives from a given point in 
the flame a certain amount of light. This same 
light, if not intercepted at A, goes on to B at a 
distance of 2 feet ; it there illuminates four 
squares of the same size as the card, and has, 
therefore, but one-fourth of its former intensity. 
If allowed to proceed to C, 3 feet, it illuminates nine such squares, and ha? 
but one-ninth of its original intensity, &c. 

Shadows. 

597. Light falling on an opaque body is intercepted 



velocity of light discovered ? State the facts and reasoning by which Roemer arrived 
nt this discovery. 595. What is the law relating to the intensity of light at different 
distances ? Give examples. 596. Illustrate this law with Fig. 228. 597. What Is 





SHADOWS. 235 

The darkness thus produced behind the opaque body is 
called its Shadow. 

598. Shadows are not all equally dark. They may be more or less illu- 
mined by reflected light or by rays from some luminous body that are not 
intercepted. Thus, if there are two lighted candles in different parts of a 
room, the shadow cast by either is less dark than if it were burning alone. 
Agaiu, the brighter the light that produces a shadow, the darker it appears 
by contrast. Hence, to compare the intensity of different lights, observe the 
shadows respectively cast at equal distances ; the one that throws the dark- 
est shadow is the brightest light. 

599. When the luminous body is larger than the opaque 
body it shines on, the latter throws a shadow smaller than 
itself; and this shadow diminishes according to the dis- 
tance of the surface on which it is thrown. 

In Fig. 229, let A be a luminous, and Fig. 229. 

B an opaque, body. B's shadow, no mat- 
ter how near the surface on which it is ^ A 
thrown, must be smaller than B itself; 
and, as the surface is removed from B, the 
shadow diminishes, till it is reduced to a point at C. 

If, on the contrary, the opaque body is the larger of the 
two, it throws a shadow greater than itself; and this shad- 
ow increases according to the distance of the surface on 
which it is thrown. 

600. The Penumbra. — Every luminous body has an in- 
finite number of points, from each of which proceeds a pen- 
cil of rays. When an opaque body is interposed, some of 
the space behind it is cut off from all the rays of the lumi- 
nous body, and this constitutes the shadow proper. Part 
of the space, however, while it Fig.230. 

is cut off from some of the rays, 
is illumined by others; this is 
called the Penumbra. 

In Fig. 230, let P be the flame of a 
candle, and AB an opaque object placed be- 
fore it. The space A B C D is not reached shadow and penitmbra. 

meant by a body's Shadow? 59S. Why are not all shadows equally dark? How 
may we compare the intensity of different lights? 599. When does a body throw a 
Shadow smaller than itself? Illustrate this law with Fig. 229. When does a body 
throw a shadow larger than itself? t500. What is meant by the Penumbra? How i% 




236 optics. 

by any ray from P, and is therefore the Shadow of A B. The space A E C, 
while it is cut off from the rays produced by the lower extremity of the flame, 
is illumined by its upper extremity ; hence it is nowhere so dark as the shad- 
ow, and becomes lighter and lighter as the line AE is approached. So the space 
B D F is cut off from the rays produced by the upper part of the flame, but 
receives those from the lower part, and is therefore partially illuminated. 
The spaces ACE, BDF, constitute the Penumbra, or imperfect shadow, 
rfAB. 

Reflection of Light. 

60 1. When light strikes an opaque body, some of it is 
absorbed, and some reflected, or thrown back into the me- 
dium from which it came. According to the Undulatory 
Theory, we should say that some of the undulations that 
strike the opaque body are brought to rest, while others 
are reproduced in the same medium with a different direc- 
tion from what they had before. 

The reflection of light is analogous to the reflected motion of an india 
rubber ball thrown against a solid surface. It is by the light irregularly re- 
flected from their surfaces that all non-luminous bodies are seen. 

Transparent surfaces, as well as opaque, reflect some of the light that 
strikes them ; otherwise, they would not be visible. We see overhanging 
objects mirrored in a stream with great distinctness, because a portion of the 
rays received from them are reflected by the water to our eyes. 

602. That branch of Optics which treats of the laws 
and principles of reflected light, is called Catoptrics. 

603. Rays that strike a body are called Incident Rays. 

604. Reflective Power of Different Surfaces.— 
Different surfaces reflect the light that strikes them in dif- 
ferent degrees. By none is the whole reflected. 

If any surface were a perfect reflector, — that is, threw back all the light 
that struck it, — the eye would fail to distinguish it. Looking at such a sur- 
face, we should see nothing but images of the bodies that produced the 
incident rays. If, for example, the moon reflected all the light it received, 
it would have the appearance of another sun. It is because there is not a 

it produced? Illustrate the mode in which the shadow and penumbra are produced, 
with Fig. 230. 601. When light strikes an opaque body, what becomes of it? Ex- 
press this according to the Undulatory Theory. To what is the reflection of light 
analogous? How are non-luminous bodies seen? Is the reflection of light con 
fined to opaque surfaces ? Prove that it is not. 602. What is Catoptrics ? 
603. What is meant by Incident Rays ? 604. What is said of the reflection of light 
from different surfaces ? If any surface were a perfeot reflector, what would be the 



REFLECTION OF LIGHT. 237 

perfect and regular reflection that the non-luminous bodies which meet the 
eye every moment are visible. 

Though incident light is never wholly reflected, yet from some surfaces it 
is thrown off with a high degree of regularity, and with its intensity dimin- 
ished comparatively little. If, for instance, we look at a good plate-glass 
mirror hung opposite to us at the end of a room, we can hardly persuade 
ourselves that there is not another apartment beyond, the counterpart of the 
one which we are in. The surface of the mirror is not seen at all, in conse- 
quence of its great reflective power. 

605. The proportion of incident light reflected depends 
on two things: — 1. The angle at which it strikes the sur- 
face. 2. The character of the surface. 

The more obliquely light strikes a surface, the greater 
is the quantity reflected. 

In Fig. 231, let C D be a surface of polished Fig. 231. 

black marble. A and B are incident beams, A 

with an intensity rated at 1,000. Let B strike 
the marble at an angle of 3 degrees, and a 
beam having an intensity of 600 will be re- 
flected. Let A strike it at an angle of 90 de- 
grees, and the reflected beam will have an intensity of only about 20. 

Light-colored and polished surfaces reflect a much 
greater proportion of incident light than dark and dull 
ones. Here again the laws of light and heat agree. 

A room with white walls is much lighter than one with black or dark- 
colored walls. A house painted some light color, or a dome covered with 
polished tin, is more readily seen from a distance than a dark wall or an or- 
dinary roof. 

606. Mirrors. — The laws of reflected light are best in- 
vestigated and explained with the aid of mirrors. 

607. Mirrors are solids with regular and polished sur- 
faces, having a high degree of reflective power. They are 
made either of some metal susceptible of a high polish, such 
as silver and steel, or of clear glass covered on the back 
with silver or a mixture of tin and mercury. A metallic 
mirror is sometimes called a Speculum (plural, specula). 



aonsequence ? What is said of the reflective power of some surfaces, such as a good 
plate-glass mirror ? 605. On what does the proportion of incident light reflected de- 
pend ? At what anglejs the most Incident light reflected ? Illustrate this with Fig. 
231. What sort of surfaces reflect the most incident light? 606. With what are the 
aws of reflected light best investigated ? 607. What are Mirrors ? Of what »re they 



238 



OPTICS. 



From glass mirrors there are two reflections ; one from the surface first 
struck, the other from the back coated with mercury. Hence two images of 
an object before the mirror are presented, the distance between them being 
equal to the thickness of the glass. But the image produced by the front 
surface is always faint ; and, when the back is well coated, the other image 
is so much superior that the faint one is entirely lost. 

608. Kinds of Mirrors. — As regards shape, mirrors are 
divided into three classes ; Plane, Concave, and Convex. 

A Plane Mirror (AB, in Fig. 232) is one that reflects 
from a flat surface, like a common looking-glass. 

A Concave Mirror (EF, in Fig. 233) is one that reflects 
from a curved surface hollowing in like the inside of the 
peel of an orange. 

A Convex Mirror (C D, in Fig. 234) is one that reflects 
from a curved surface rounding out like the outside of an 
orange. 

A concave mirror polished on both sides becomes a convex mirror when 
its opposite side is presented to the incident rays. 

609. Great Law of Reflected Light. — The law of 
reflected light is like that of reflected motion : — The angle 
of reflection is always equal to the angle of incidence. This 
law holds good whether the reflecting surface is plane, con- 
cave, or convex. 

Fig. 232. Fig. 233. Fig. 234. 

P 

I ! H 





Figs. 232, 233, 234, illustrate this law. In each Figure, I represents the 
incident ray, R the reflected ray, and P a perpendicular. I Q P, the angle 
which the incident ray makes with the perpendicular, is called the angle of 
incidence. R Q P, the angle which the reflected ray makes with the same 
perpendicular, is the angle of reflection. From every surface, whatever its 
form, the incident ray is thrown off in such a way as to make the angle of 
reflection equal to the angle of incidence. 



made ? What is a Speculum ? How many reflections are there from glass mirrors ? 
How are they produced ? What is said of the images formed ? 608. As regards 
shape, how are mirrors divided ? What is a Plane Mirror? What is a Concave Mir- 
ror ? What is a Convex Mirror ? How may a concave mirror polished on both sides 
be made * convex mirror? 609. State the law of reflected light. Illustrate this with 



FORMATION OF IMAGES. 



239 



610. From these Figures it is obvious that an object which would not 
otherwise be visible can be seen by reflection from a mirror. Thus, let the 
upper part of P Q represent an opaque screen, I an object on one side of it, 
and R the eye of an observer on the other. I is not visible to a person at R 
looking directly at it, on account of the interposition of the screen ; but, as 
the angle of reflection is always equal to the angle of incidence, it can be 
seen from R by looking at the mirror. 

611. Images. — By the Image of an object is meant a 
luminous picture of it formed by rays proceeding from its 
different points. An image is said to be inverted when it 
represents its object as upside down, — that is, with its low- 
est part uppermost. 

Fig. 235. 




Fig. 235 illustrates the formation of an image. R B represents a soldier 
with a red coat and blue trowsers standing in strong sunlight opposite 
the white wall W. Let the shutters S S be thrown open, and not only the 
light reflected from the person of the soldier, but also other rays, enter the 
apartment, making its light a mixture of all colors, or white, in which the 
red and the blue tinge of the dress are lost, and no image is formed. Now let 
the shutters S S be closed, leaving at A an exceedingly small aperture, through 
which the rays reflected from the figure are allowed to reach the wall. As 
light is propagated in straight lines, the ray R will strike the wall at r, B at 
b, and I at i. The image will therefore be inverted ; and, as each ray retains 
its color, the coat will remain red and the trowsers blue. This experiment 
confirms two principles already stated : — 1. That every ray moves in a 
straight line ; 2. That an infinite number of rays may cross each other with- 
out interfering with the effect which each would separately have. 

612. Images formed by apertures are always inverted. 



the Figures. 610. What is obvious from these Figures ? 611. What is meant by the 
Image of an object? When is an image said to be inverted? With Fig. 235, illus- 
trate the formation of an image. What two principles does this experiment confirm ? 



240 



OPTICS. 



613. Reflection from Plane Mirrors. — Plane mir- 
rors do not alter the relative direction of incident rays. If 
the incident rays are parallel, they will remain parallel after 
reflection ; if divergent, they will continue to diverge ; if 
convergent, they will continue to converge. 

614. Objects seen in a plane mirror seem to lie in the 
direction of the reflected rays that meet the eye, and to be 
as far behind the mirror as they really are in front of it. 
These principles are illustrated with Fig. 236. 

A B is a plane mirror. , C, D, are parallel rays striking 
its surface. They are reflected in parallel lines to c, d ; 
and to an observer at those points will appear to come 
from G, H, as far behind the mirror as C, D, are in front 
of it. 

E is a diverging pencil. After reflection, its rays con- 
tinue to diverge to e, e, e; and to an observer there they 
appear to diverge in unbroken straight lines from the poin^ 
I, as far behind the mirror as E is before it. 

F, F, F, represent converging rays. After reflection, 
they continue to converge, and meet at the point /. An 
observer aty would suppose them to come in unbroken 
lines from J, J, J, as far behind the mirror as F, F, F, are 
in front of it. 

615. When we walk towards a looking-glass, our image 
seems to advance towards us ; and when we recede from 
it, the image also recedes. The image always appears to be the same dis- 
tance from the mirror that the object is. 

616. The angle of reflection being equal to the angle of 
incidence, it follows that a person may see his whole figure 
reflected from a mirror whose 
length is but half his own height. 
In Fig. 237, CD represents a 
man standing before the mirror 
AB. The incident ray from the 
head C strikes the mirror perpen- 
dicularly, is reflected in the same line, and appears to come 




Fig. 237. 
A 



612. What kind of images are formed by apertures ? 613. What effect have plane 
mirrors on the relative direction of incident rays? 614. How do objects seen in a 
plane mirror seem to lie? With Fig. 236, illustrate the reflection of parallel, diverg- 
ing, and converging rays from a plane mirror. 615. When we approach and recede 
from a looking-glass, what phenomena are presented ? 616. How is it that a person 
can see his whole figure reflected from a mirror whose length is but half his height ? 



REFLECTION FROM PLANE MIRRORS. 241 

from E. The ray from his foot D strikes the mirror at B, 
is reflected at an equal angle to his eye, and appears to 
come in an unbroken line from F. The extremities of his 
person being seen, the intermediate parts are also visible, 
forming a complete image. 

617. Images formed by Plane Mirrors. — The size of 
images formed by plane mirrors is not changed, except so 
far as they seem smaller in consequence of their appareLt 
distance behind the mirror. 

618. As the image faces the opposite way from the object, if the mirror is 
vertical (that is, perpendicular to the floor), the right side of the object will 
be the left of the image, and the left side of the object the right of the image. 
If a person stands before a mirror with a book in his right hand, the book 
seems to be in the left hand of his image ; and, if he brings the printed page 
near the mirror, he can not read it, for the reflection turns about both letters 
and words, side for side. 

Place the same plane mirror in a horizontal position (that is, lay it on the 
floor with its face up), and the image, which before simply had its sides 
transposed, now becomes inverted, or seems to stand on its head. On the 
same principle, a tree or other object reflected from the surface of a pond, is 
inverted. 

619. The Kaleidoscope. — When an object is placed be- 
tween two parallel plane mirrors, each produces an image 
of its own, and reproduces the image reflected to it from 
the other. This image of an image is again reflected by 
each to the other, and thus a series of images is produced, 
till the rays become so faint by successive reflections as to 
be no longer discernible. 

When the mirrors are placed at right angles to each 
other, an object between them forms three images, — one 
produced by each separately, and one by a twofold reflec- 
tion from both. Placed so as to form with each other an 
angle of 60 degrees, the two mirrors will produce five im- 
ages ; at 45 degrees, seven. 

This principle is applied in the Kaleidoscope [ka-li'-do- 
scope], a beautiful toy invented by Sir David Brewster. 

617. What is said of the size of images formed by plane mirrors ? 618. If the mirror 
is vertical, how does the image differ from the object ? How, if the mirror is horizon- 
tal? 619. What takes place when an object is placed between two parallel plane 
mirrors? How many images are formed when the mirrors are placed at right angled 

1« 



242 



OPTICS. 



Fig. 238. 



620. The kaleidoscope consists of two narrow strips of glass running 
lengthwise through a tube, and forming with each other an angle of 60 or 45 
degrees. One end of the tube, to which the eye is to be applied, is covered 
with clear glass. The other end terminates in a cell formed by two parallel 
pieces of glass an eighth of an inch apart, the outer one of which is ground 
to prevent external objects from marring the effect. This cell contains beads 
or small pieces of glass of different colors, free to move among themselves. 
On applying an eye to the tube, we see the objects in the cell multiplied by 
repeated reflections from the mirrors, and symmetrically arranged, with their 
images, around a common centre. By shaking the tube, we bring the ob- 
jects into new relative positions, and have new combinations presented. 

621. The Magic Perspective. — By arranging four plane 
mirrors as represented in Fig. 238, a person is enabled to 
see an object by looking directly towards it, though an 
opaque screen is interposed. 

A rectangular box is bent 
four times at right angles; 
and in each of these angles 
is placed a piece of looking- 
glass, B, C, D, E, at such 
an inclination that the inci- 
dent ray may strike it at an 
angle of 45 degrees. Any 
object opposite the aperture 
A is visible to an eye ap- 
plied at the other extremity, though an opaque screen be placed between the 
arms of the instrument. The rays from the object first strike B at an angle 
of 45 degrees, and are reflected at the same angle to C, thence to D, thence to 
E, and finally to the observer's eye. The inventor of this instrument recom- 
mended its use in time of war, for discovering an enemy's movements with- 
out any exposure of the observer's person. It is more commonly used, how- 
ever, by itinerant showmen, who for a penny allow the curious to read through 
a brick. 

622. Reflection from Concave Mirrors. — In gen- 
eral, the effect of concave mirrors is to make incident rays 
more convergent or less divergent. In most cases, the im- 
ages they produce appear in front of them. 

623. Parallel rays striking a concave mirror are made 
to converge to a point called the Principal Focus. This 

io each other ? How many, when they form an angle of 60 degrees ? Of 45 degrees ? 
In what is this principle applied ? 620. Describe the Kaleidoscope. 621. How is a 
person enabled to see an object by looking towards it, though an opaque screen is in- 
terposed ? Describe the Magic Perspective. By whom is it commonly used ? 
622. What is the general effect of concave mirrors ? "What is said of the images they 




THE MAGIO PERSPECTIVE. 



REFLECTION FROM CONCAVE MIRRORS. 243 

point is half way between the surface of the mirror and the 
centre of the sphere which the mirror would form if it were 
extended with uniform curvature. 

In Fig. 239, let A E B be a concave mir- Fig. 239. 

ror, forming part of the surface of a sphere, 
of which C is the centre. The parallel rays 
d, e,f, g, h, are reflected to the principal fo- 




cus F, midway between the surface and the E \~^y~ ^ 

centre C. X^T~ •? 

Not only is light concentrated at the fo- 
cus, but also heat, as we had occasion to 

note in § 476. Tinder, wood, or any other combustible material, is readily 
ignited, and with a combination of such mirrors the most intense heat can be 
produced. Hence concave mirrors are sometimes called Burning Glasses. 

624. Converging rays reflected from a concave mirror 
are made to converge more. 

625. Diverging rays reflected from concave mirrors are 
differently affected according to the position of the point 
from which they diverge. 

626. Diverging rays starting from the principal focus 
are made parallel. This is obvious from Fig. 239. The 
rays diverging from F, after striking the mirror, are re- 
flected in parallel lines to d, e, f, g, h. 

This principle is turned to account in light-houses. The light is placed in 
the focus of a concave mirror, and its rays are reflected in parallel lines from 
every point of the mirror's surface. No image of the light is produced, but 
the whole surface of the mirror appears illuminated. 

627. Diverging rays coming from a point between the 
principal focus and the mirror, become less divergent after 
reflection. An object in such a position forms an image 
larger than itself, which seems to be situated behind the 
mirror. 

628. Diverging rays coming from a point between the 



produce ? 623. What effect has a concave mirror on parallel rays that strike it ? 
How is the principal focus situated ? Illustrate this effect with Fig. 239. What are 
concave mirrors sometimes called, and why ? 624. What is the effect of concave mir- 
rors on converging rays ? 626. What is the effect of concave mirrors on diverging 
rays starting from the principal focus? How is this principle turned to account? 

627. What effect have concave mirrors on diverging rays coming from a point be- 
tween the principal focus and the mirror? What kind of an image is formed? 

628. What effect have concave mirrors on rays diverging from a point between the 



244 optics. 

principal focus and the centre, converge, after reflection, 
to a focus on the other side of the centre. An inverted 
image will there be visible, suspended in the air. This im- 
age is made more distinct, and its effect greatly increased, 
by causing a cloud of thin bluish smoke to rise about the 
spot from a chafing-dish placed beneath. . 

By concealing with screens the mirror, the object, and the light that illu- 
mines it, and allowing the reflected rays to pass through an aperture, we may- 
give the image all the appearance of reality. The observer beholds delicious 
fruit hanging in the air without any visible support, and can hardly convince 
himself that it is a delusion, even when he tries to grasp it without success. 
He sees a pail full of water standing bottom upward without spilling its 
contents, and men with every semblance of life walking on their heads. It 
was with apparatus of this kind that the pretended magicians of the Middle 
Ages wrought many of their miracles, terrifying the uninitiated with sudden 
apparitions of skulls, drawn swords, skeletons, ghosts, &c. 

629. Diverging rays coming from the centre are reflect- 
ed by a concave mirror back to the same point. Here, as 
in all other cases, the angle of reflection is equal to the an- 
gle of incidence. Striking the surface at right angles, they 
are reflected at right angles back to the centre. 

630. Diverging rays coming from a point beyond the 
centre, after reflection by a concave mirror, converge to a 
point on the other side of the centre. In this case, the im- 
age is inverted and smaller than the object. 

631. Reflection by Convex Mirrors. — In general, the 
effect of convex mirrors is to make incident rays more di- 
vergent or less convergent. The images they produce, like 
those of plane mirrors, seem to stand behind them, and are 
generally smaller than the objects they represent. 

632. Parallel rays striking a convex mirror are made to 
diverge, as if they proceeded from a point on the opposite 
side of the mirror, called the Virtual Focus. This point is 

principal focus and the centre ? "What sort of an image is formed ? How is the image 
made more distinct : How may wonderful effects be produced with this mirror? By 
whom was apparatus of this kind employed? 629. What is the effect of concave mir- 
rors on diverging rays coming from the centre ? 630. What is their effect on diverg- 
ing rays coming from a point beyond the centre ? In this case, what kind of an image 
is produced? 631. What is the general effect of convex mirrors ? What is said of 
toe images th^y proouoe? 632. What is the effect of a convex mirror on parallel 



REFLECTION BY CONVEX MIRRORS. 245 

half way between the mirror and the centre of the sphere 
which the mirror would form, if it were extended with uni- 
form curvature. 

In Fig. 240, let AB represent a Fig. 240. 

convex mirror forming part of the f 

surface of a sphere, of which C is the A / y'il 

centre. The parallel rays a, b, c, d, 
diverge after reflection to/, g, c, h, i, 




as if they had come from the virtual g— - -£-'- — — — c 

focus F on the other side of the mir- \ ^^//- tf, 

ror. F is half way between the mir- 
ror and its centre C. \ ^Xy, 

633. Diverging rays fall- 
ing on a convex mirror are made more divergent by reflec- 
tion. Converging rays are made less convergent, in some 
cases even becoming parallel. 

Refraction of Light. 

634. When light strikes a transparent body, some of it 
is reflected and makes the body visible. The rest enters 
the body, and is partly absorbed and partly transmitted 
through it. According to the undulatory theory, we should 
say that some of the undulations that strike the transparent 
body are reproduced in the same medium with a change of 
direction, while others are brought to rest within the body, 
and others again are transmitted through it with certain 
modifications. 

We have treated of that portion of the light which is 
reflected ; we must now look at that which enters the trans- 
parent body. 

635. "When a boy rowing a boat brings his oar into the water, it no longer 
looks straight, but broken at the point where it enters. The same appear- 
ance is presented when he plunges a spoon or cane obliquely in a pail of wa- 
ter. On taking out the oar, the spoon, and the cane, they look perfectly 
straight again. It is evident, therefore, that the rays coming from the parts 

rays ? Where does the virtual focus lie ? Illustrate the effect of convex mirrors on 
parallel rays, with Fig. 240. 633. What is the effect of convex mirrors on diverging 
rays? On converging rays? 634. When light strikes a transparent body, what be- 
comes of it ? Express this according to the Undulatory Theory. 635. Give some fa- 
miliar examples which prove that rays are bent on passing from one medium to an« 



246 optics. 

immersed are turned from their course on entering the air, so that the points 
from which they come appear to lie where they do not really lie. Rays thus 
turned from their course are said to be refracted. 

636. Refraction is that change of direction which a ray 
of light experiences on passing obliquely from one medium 
to another. 

For an example, see the ray A in Fig. 241. If there were no water in the 
vessel, it would go on in a straight line to B ; when the vessel is filled, it is 
refracted to C. 

637. That branch of Optics which treats of the laws and 
principles of refracted light, is called Dioptrics. 

638. Refractive Power op Different Media. — All 
media do not have the same refractive power. Rays of 
light falling from the air on water, alcohol, glass, and ice, 
are turned from their course in different degrees by each. 

A medium that has great refractive power is said to be 
dense; one that has but little, is called rare. The terms 
dense and rare, therefore, applied to media in Optics, have 
a different meaning from that which they convey in other 
departments of Natural Philosophy. 

As a general rule, those media are the densest that have the greatest spe- 
cific gravity ; and, of media having about the same specific gravity, the most 
inflammable is the densest. The following substances are arranged accord- 
ing to their refractive power, chromate of lead, a transparent solid, being the 
densest: — Chromate of lead, diamond, phosphorus, sulphur, mother-of-pearl, 
quartz, amber, plate-glass, olive oil, alcohol, water, ice, air, oxygen, hy- 
drogen. 

639. Laws of Refracted Light. — 1. In a uniform 
medium, there is no refraction. It is only on passing from 
one medium {or stratum of a medium) to another, that a 
ray is turned from its course. 

2. Only such rays as enter a medium obliquely are re- 
fracted, — not such as enter at right angles. 

3. When a ray passes obliquely from a rarer to a denser 

other. What term is applied to such rays? 636. What is Refraction? Illustrate this 
definition with Fig. 241. 637. What is Dioptrics? 638 What is said of the refractive 
power of different media ? What is a Dense Medium ? What is a Rare Medium ? 
What is said of the meaning of the terms dense and rare in Optics? As a general 
rule, what media are the densest ? Mention some substances in the order of their 
refractive power ? 639. What is the first law of refracted light? The second ? The 



REFRACTION. 



241 




medium, it is refracted towards a line perpendicular to 
the surface. In Fig. 241, let the ray A pass from air, a 
rarer medium, into water, a denser medium, and instead of 
going on in a straight line to B, it will be 
refracted to C, nearer the perpendicular. 

4. When a ray passes from a denser me- 
dium into a rarer, it is refracted from the 
perpendicular. In Fig. 241, let the ray B 
pass obliquely from water into air, and in- 
stead of going on in a straight line to A, it 
will be refracted to D, farther from the perpendicular. 

640. An interesting experiment which every pupil may perform for him- 
self, admirably illustrates refraction, and proves the last law to be true. 
Place a coin on the bottom of an empty vessel (see Fig. 242. 

Fig. 242), and fix the eye in such a position that 
it just misses seeing it on account of the vessel's 
side coming between. Keep the eye there, and 
let water be poured in ; the coin will then become 
visible, the rays from its surface being refracted 
so as to meet the eye. The coin will appear to lie 
at N, some distance above the bottom of the ves- 
sel ; because the rays from it that last meet the eye, if continued in straight 
lines, would go on to that point. 

The change caused by refraction in the apparent position of an object 
often misleads persons standing on the bank of a sheet of water as to its 
depth. Objects on the bottom seem to be several feet nearer the surface than 
they are, and bathers, deceived by the appearance, venture beyond their 
depth and are drowned. 

641. Atmospheric Refraction. — Rays from the heav- 
enly bodies, on entering our atmosphere obliquely from a 
rarer medium, are refracted towards the perpendicular. 
Hence we never see these bodies in their real position, ex- 
cept when they are directly over head. 

The sun is visible to us some time before he really rises above the horizon, 
and remains visible at night after he has sunk below it. We owe our twi- 
light to successive reflections and refractions of his rays by atmospheric 
strata of different densities, after he has disappeared. 




third ? The fourth ? Illustrate the third and the fourth law with Fig. 241. 640. What 
interesting experiment illustrates refraction ? How are persons standing on the bank 
of a sheet of water often deceived? 641. When do we see the heavenly bodies in 
their real position ? Why, at other times, do we not see them in their real position ? 



248 optics. 

642. Mirage. — Different strata of the atmosphere differ 
in their refractive power. Accordingly, rays from an ob- 
ject below the horizon (that is, concealed from us by the 
roundness of the earth) may, under peculiar circumstances, 
by successive refractions through different strata, be made 
to describe a curve to our eyes, and will in that case ap- 
pear to come from a distant point in the air lying in the 
direction of the line described by the ray as it entered the 
eye. Such is the origin of the phenomenon called Mirage 
\me-rahzK\ 

Mirage is the appearance in the air of an erect or in- 
verted image of some distant object which is itself invisible. 
It is most frequently seen on the water, but has also ap* 
peared to persons travelling through deserts, with such viv- 
idness as to make them believe that they saw trees and 
springs before them in the distance. 

Mirage is sometimes remarkably distinct at sea. Captain Scoresby, on 
one occasion, in a whaling-ship, recognized his father's vessel, when distant 
from him more than 30 miles (and consequently below the horizon), by its 
inverted image in the air, though he did not previously know that it was 
cruising in that part of the ocean. Another notable case occurred on the 
coast of Sussex, England. Cliffs were distinctly seen in the air ; and the 
sailors, crowding to the beach, recognized different parts of the French shore, 
distant from 40 to 50 miles. These phenomena are comparatively frequent 
in the Strait of Messina, and as there exhibited have been called Fata Mor- 
gana [fah'-tah mor-gaTi'-na7i\. 

643. Refraction by Prisms and Lenses. — Prisms and 
lenses are much used in experimenting on light and in the 
construction of optical instruments. 

Fig 243 644. Prisms. — A Prism (see Fig. 243) 

is a solid piece of glass, having for its sides 



<F=^ 



three plane surfaces and for its ends two 
a pkism. equal and parallel triangles. 

645. A ray of light falling on a prism must pass through 
two of its surfaces. If it strike both of them obliquely, it 

To what do we owe our twilight ? 642. Explain how an object below the horizon is 
Tendered visible. What phenomenon is thus produced ? What is Mirage ? Where 
is it seen ? What case of mirage is recorded by Captain Scoresby ? What other nota- 
ble case is mentioned ? Where are these phenomena frequent ? 643. What are much 
used in experimenting on light? 644. What is a Prism ? 645. What is the effect of 



REFRACTION BY PRISMS AND LENSES. 



249 



Fig. 244. 



r^t 




will be twice refracted ; if it strike one surface perpendic- 
ularly and the other obliquely, it will be refracted but once. 
In either case, the object from which it conies will appear 
to lie in a position more or less removed from its real one. 

Fig. 244 shows the refractive effect of a prism. 
A ray from E, entering the prism ABC, from 
air, a rarer medium, is refracted to D, and on 
passing back into the rarer medium, at that point 
is refracted to the eye. The object from which it 
comes appears to lie at F, in tbe direction from 
which the ray entered the eye. Had there been 

but one refraction, it would still have appeared elevated above its real posi- 
tion, but not so much. 

646. Lenses. — A lens is a transparent body which has 
two polished surfaces, either both curved or one curved and 
the other plane. The general effect of lenses is to refract 
rays of light, and magnify or diminish objects seen through 
them. They are generally made of glass ; but in specta- 
cles rock crystal is sometimes used instead of glass, because 
it 13 harder and less easily scratched. 

647. Classes of Lenses.— Lenses are divided into six 
classes according to their shape. Fig. 245 shows these six 
classes. The name of each is given on one side, and a de- 
scription of it on the other. 

Fig. 245. 



Double Convex Lens. 
Plano-convex Lens. 

MENI8CTT8. 

Double Concave Lens. 
Plano-concave Lens. 
Concavo-convex Lens. 




Both sides convex. 



One side convex, the other plane. 

One side convex, the other concavft. 
Thickest in the middle. 



Both 



One side concave, the other plane. 

One side concave, the other convex. 
Of uniform thickness, or thickest at the 
ends. 



& prism on a ray of light ? Show this effect with Fig. 244. 646. What is a lens ? 
What is the general effect of lenses ? Of what are they made ? 647. Into how many 
classes are lenses divided ? Name them. Describe the Double Convex Lens. The 
Plano-convex. The Meniscus. The Double Concave Lens. The Plano-concave. 



250 OPTICS. 

The first three of the above lenses, which are thickest in the middle, are 
called Convex Lenses, and their effect is to make rays passing through them 
incline more towards each other. The next two (the double concave and 
plano-concave) which are thinnest in the middle, are called Concave Lenses, 
and their effect is to make rays passing through them incline farther from 
each other. 

The concavo-convex lens, when its two surfaces are parallel (as in the 
above Figure) does not change the direction of rays passing through it, for 
the convergent effect of the convex surface is nullified by the divergent effect 
of the concave surface. When the convex surface has a greater curvature 
than the concave, this lens becomes a meniscus. "When the concave surface 
has the greater curvature, it becomes a concave lens, and participates in the 
properties of that class. 

648. Refraction by Convex Lenses. — The general effect 
of convex lenses is threefold: — 1. They make rays passing 
through thera incline more towards each other than before. 
2. They enable us to see objects which are invisible to the 
naked eye on account of their distance. 3. They magnify 
objects seen through them. 

649. A double convex lens of glass, with sides equally 
convex, brings parallel rays passing through it to a focus at 
the centre of the sphere, of which the surface of the lens 
first struck by the rays forms a part. This is shown in Fig. 
246. Converging rays would be brought to a focus be- 
tween the centre and the lens ; diverging rays, on the other 
side of the centre. 

Fig. 246. Fig. 247. 





The Concavo-convex. What are the first three of these lenses called ? What Is their 
effect ? What are the double concave and the plano-concave lens called ? What is 
their effect ? What is the effect of the concavo-convex lens, when its two surfaces are 
parallel? When the convex surface has a greater curvature than the concave? 
When the concave surface has a greater curvature than the convex? 648. What is 
the general effect of convex lenses ? 649. What is the effect of a double convex glass 
lens on parallel rays passing through it ? On converging rays ? On diverging rays? 






REFRACTION BY LENSES. 251 

A plano-convex lens brings parallel rays to a focus at a 
distance from the lens about equal to the diameter of the 
sphere of which the convex surface of the lens forms a part. 
This is shown in Fig. 247. 

650. Convex lenses collect heat as well as light at their focus. Hence 
they are sometimes called Burning Glasses. Hold an old person's eye-glass 
in the sun-shine a short distance from your hand. A bright spot of light 
marks the focus, and the heat at that point soon becomes too great to be 
borne. All the rays that fall on the surface of the lens being concentrated 
m this one point, the heat at the focus is as many times greater than the heat 
of ordinary sun-Ught as the area of the lens is greater than the area of the fo- 
cus. If the area of the lens be 100 square inches, and that of the focus V4 of 
an inch, the ordinary heat of the sun will be increased 400 times. 

651. The second effect of convex lenses follows from the 
first. Light, it will be remembered, diminishes in intensity 
according to the square of the distance from the luminous 
body ; hence rays from exceedingly remote stars become 
so faint by the time they reach the eye as not to produce 
the sensation of vision. A convex glass concentrates a great 
number of these faint rays, and thus renders the distant 
object visible to an eye placed at its focus. 

652. The third effect of convex lenses is to magnify ob- 
jects seen through them. Hence they are sometimes called 
Magnifying Glasses. The glasses used by old persons, as 
well as by engravers and others who have to deal with mi- 
nute objects, are convex lenses, 

653. Refraction by Concave Lenses. — The effects of 
concave lenses are opposite to those of convex. 1. They 
make rays passing through them incline farther from each 
other, 2. They diminish objects seen through them. 

654. All the above laws relating to prisms and lenses apply to rays pass- 
ing into them from a rarer medium, such as air. If they come from a denser 
medium, the results will be reversed, — convex lenses will have a diverging 
and diminishing effect, while concave lenses will have a converging and 
magnifying effect. 



What 13 the effect of a plano-convex Ions on parallel rays ? 650. What are conves 
tenses sometime* called, and why ? How may their concentration of heat be shown ? 
How does the heat at the focns compare with that of ordinary sun-light ? 651. Show 
bow a convex lens enables us to see distant heavenly bodies that would otherwise be 
Invisible. 652. What is the third effect of convex lenses ? What are they sometimes 



252 



OPTICS. 






Fig. 248. 




THE MULTIPLYING 
GLASS. 



655. Glasses with Parallel Surfaces. — When rays pass through a refracting 
medium having parallel surfaces, they leave it, not exactly in the same line, 
but in a direction parallel to that in which they entered it. The last refrac- 
tion nullifies the change of direction produced by the first. Hence we see 
objects through a pane of window-glass very nearly in their real position. Ir- 
regularities in the glass cause objects seen through it to look distorted. 

656. The Multiplying Glass. — If a plano-convex lens 
have its convex surface ground into several flat surfaces, an 
object seen through it will be multiplied as many times as 
there are flat surfaces. 

In Fig. 248, A B represents a multiplying glass, and 
D an object viewed through it. The ray D C, striking 
both surfaces perpendicularly, reaches the eye without 
refraction ; but D I and D F, falling obliquely, suffer 
two refractions, which bring them also to the eye at 
the focus. As objects are always seen in the direction 
in which their rays enter the eye, three objects like D 
will be visible : one at D, in its real position ; .the 
others, in the direction of the dotted lines, at G and H. 

657. Double Refraction. — Certain 
substances (chiefly minerals) have the prop- 
erty of causing rays which pass through them to take two 
distinct paths, and thus produce two images. This phe- 
nomenon is called Double Refraction. 

Fig. 249. A crystal of carbonate of lime, 

commonly called Iceland Spar, is 
one of the best substances for ex- 
hibiting double refraction. Let it be 
placed over a piece of paper con- 
taining lines, and each line will be 
seen double, as shown in Fig. 249. 

Keeping the same side on the 
paper, and turning the crystal round 
on its axis, we find that the double 
lines continue parallel, but that the 
distance between them varies, — diminishing till they coincide, then increas- 
ing; then diminishing till they coincide again, and then once more increas- 

called in consequence? 653. What are the general effects of concave lenses ? 654. In 
what case do the above laws relating to prisms and lenses apply ? Suppose the rays 
pass into them from a denser medium, what will be the result ? 655. What effect hafc 
& refracting medium with parallel surfaces on incident rays ? How do we see objects 
through a pane of window-glass ? 656. How is the multiplying glass formed ? How 
many times is an object seen through it multiplied? Show this with Fig. 248. 
657. What is Double Eefraction ? How is it exhibited with Iceland spar ? What phe- 




m 

MM1-. 



POLARIZATION OF LIGHT. 253 

ing. During each revolution of the crystal, the lines will coincide twice. 
A single pencil of rays is thus refracted into two distinct pencils, one of 
which, following the usual law of refraction, is called the Ordinary Pencil, 
while the other, deviating from that law, is called the Extraordinary Pencil. 

Polarization of Light. 

658. Light is said to be polarized, when, on being re- 
flected or refracted by a surface or medium, which it strikes 
at a certain angle, its capacity for reflection or refraction 
a second time depends on the position of the new surface 
or medium on which it strikes. 

Let A and B (Fig. 250) be two tubes open at both Fig. 250. 

ends, and so adjusted to each other that B turns stiff- 
ly within A. In each tube fix a piece of polished 
glass, M, N, roughened and blackened on the back, 
so as to form an angle of 33 degrees with the axis of <? 
the tubes. Bring the instrument into such a position 

that a ray from a luminous body, falling on M, may be reflected along the 
axis and strike N. Now, keeping the tube A stationary, turn within it the 
tube B, carrying the reflector N. The reflection from N will be seen to keep 
varying in intensity. In the two positions in which tbe reflection from K 
is in the same plane as that traversed by the ray before and after its first 
reflection, the ray is reflected most brightly ; at the positions midway 
between these, one of which is shown in Fig. 250, there is no reflection at 
all. We express this by saying that the light reflected from M is polarized. 

659. The polarizing angle, — that is, the angle which the 
incident ray must make with a perpendicular to the first 
reflecting surface, in order to be polarized, — is different in 
the case of different substances. For glass, it is about 57°. 

660. If a polarized ray be received on a crystal of Ice* 
land spar, there will be but a single refraction. 

661. Light is polarized by reflection at a certain angle, as we have just 
seen ; by transmission through substances that have the property of double 
refraction, — through some imperfectly crystallized substances, such as agate, 
mother-of-pearl, &c, — and also through a sufficient number of uncrystallized 
plates. However produced, polarized light always has the same properties. 
Its phenomena are striking, and seem to prove the truth of the undulatorj 

liomena are presented as the crystal is turned around ? What are the two pencils 
presented to the eye called ? 65S. When is light said to be polarized? Illustrate the 
polarization of light with Fig. 25(X 659. What is meant by the polarizing angle? 
What is this angle in the case of glass? 660. If a polarized ray is received on a crys- 
tal of Iceland spar, what follows? 661. Mention the different ways in which ]ightis 
polarized. What is said of the properties and phenomena of polarized light, how< 



254 OPTICS. 

theory. It is thought that the undulations of ether ordinarily take place in 
planes perpendicular to the direction in which they are propagated ; but 
that, when light is polarized, they take place in planes parallel to this direc- 
tion. At certain angles, the undulations, thus changed from their usual di- 
rection, are reproduced or transmitted by the second reflecting or refracting 
surface, and reach the eye ; but, at an angle of 90 degrees, they are 
stopped, and the sensation of vision is not produced. 

662. The mineral called Tourmaline \toor'-ma-leen\ pos- 
sesses the property of polarizing light in a high degree. It 
is cut into plates one-twentieth of an inch thick, which are 
fixed between plates of glass for convenience of use. If we 
look at the sun through such a plate, we shall find that most 
of the light is transmitted. Place a second plate behind 
the first, with its axis in the same direction, and the light 
will still be transmitted ; but turn the second plate quarter- 
way round, and no light will pass "through. 

663. Some crystals viewed by polarized light, exhibit systems of beautiful 
rings, like those shown in Fig. 251. Plates of the mineral called Selenite, 

Fig. 251. 



flllHHt 

-^^i£ik% ,■■ ,>',i' , :, 1 i' ,, :; 1 .iii'jiiiiiiiiiri. \\x\\ 



1 I 






§§ 



'wrm 



m 1 ; 



mf \**SSr 



sW 



bearing different designs, placed so as to be seen by polarized light, display 
the most gorgeous coloring, and may be made to undergo remarkable and 
beautiful changes by causing one of the reflecting surfaces to revolve. 

Chromatics. 

664. Chromatics is that branch of Optics which treats 
of colors. 

ever it is produced? Explain the polarization of light according to the undulatory 
theory. 662. What mineral possesses the property of polarizing light in a high de- 
gree ? How is tourmaline prepared ? What experiment may be performed with tour- 
maline plates ? 663. What phenomena are seen when certain crystals are viewed 



THE SOLAR SPECTRUM. 



255 



665. The Solar Spectrum. — If a ray from the sun be 
admitted into a dark room through a small aperture, it will 
form a circular spot of white light on the surface receiving 
it. But if, after entering the room, it be received on a 
prism, as shown in Fig. 252, it will be decomposed into 

Fig. 252. 




THE SOLAR SPECTRUM. 



seven different colors. When made to fall on a white sur- 
face, these seven colors are distinctly seen, covering an 
oblong space, which is called the Solar Spectrum (plural, 
spectra). They are known as the Primary Colors, and in 
every spectrum they are arranged in the order shown in 
the Figure. By combining the primary colors in different 
proportions, other colors are produced. 

The seven colors, it will be observed, do not occupy 
equal spaces of the spectrum. Yiolet covers the greatest 
part, more than one-fifth of the whole ; and orange the 
least, less than one-thirteenth of the whole. 

666. Ordinary sun-light (and all white light) is therefore composed of 
seven colors combined in different proportions. In further proof of this, we 
may re-unite the seven primary colors of the spectrum, and we shall have 
simply a small circular spot of white light. To re-unite the colors, we may 
"receive the spectrum on a concave mirror or double convex lens, which brings 
together at its focus the parts of the decomposed ray. Or, we may receive 
the spectrum on another prism placed in contact with the first, as shown 
in Fig. 252. In either case, we have the same circular spot of white light 
that would have been formed if the ray had not been decomposed at all. 



Oy polarized light ? When plates of selenite are viewed by polarized light? 
664. What is Chromatics ? 665. Describe the solar spectrum, and the way in which 
it is formed. Name the seven primary colors in order. How are the other col- 
ors produced ? Which color occupies most of the spectrum, and which the least ? 
666. Of what, then, is all white light composed? What further proof have we 



256 optics. 

We may produce white light by combining the seven primary colors In 
another way. Divide the surface of a circular card into seven parts propor- 
tioned to each other as the spaces which the different colors occupy in the 
spectrum, and paint them the corresponding shades. Then cause the card 
to revolve rapidly. No separate color will be visible, but the whole card 
will look white. 

667. A prism decomposes white light into its seven component parts, be- 
cause these parts are refracted differently, some more and some less. It will 
be observed that red, which occupies the lowest part of the spectrum, is 
turned from its course the least ; orange, a little more ; yellow, still more ; 
then green ; then blue ; then indigo ; while violet, which is at the top of the 
spectrum, is refracted the most. The colors, therefore, have different de- 
grees of refrangibility. This fact was discovered by Sir Isaac Newton. 

668. Difference of Color, explained. — According 
to the Undulatory Theory, the color of light depends on 
the size of the minute waves that produce it. The undula- 
tions that excite in the eye the sensation of red light are 
each 3- T £oo °f an mcn m breadth ; those that produce vio^ 
let, eoooo 5 while the intermediate colors are produced by 
undulations varying between these limits. 

669. Color is not a property inherent in bodies, but in 
the light that they reflect. A non-luminous body seems to 
be whatever color it reflects to the eye. 

An object lying in green light, looks green ; in red light, red, <fec. This 
is because green or red is the only light that falls upon it, and therefore it 
can reflect no other to the eye. A body seen by ordinary light looks green, 
when it absorbs all or most of the other colors of the spectrum, and reflects 
or transmits green alone. It looks red when it absorbs the other colors, and 
reflects or transmits red, &c. It looks white, when it does not decompose the 
light that falls on it, but reflects all the colors combined. It looks black, 
when it absorbs nearly all the light that falls on it, and does not reflect anv 
particular color in preference to the rest. 

670. What colors a substance absorbs and what it reflects, depends chiefly 
on its structure. The particles of some bodies are so arranged as to have 
a peculiar affinity for certain colors ; these they absorb, reflecting the rest. 

of this ? How may we re-unite the seven primary colors ? What other mode is 
there of doing this ? 667. To what is it owing that a prism decomposes white light 
into its seven component parts ? By whom was this fact discovered ? 668. Ac- 
cording to the Undulatory Theory, on what does the color of light depend? What 
is the difference in the undulations that respectively produce red and violet light? 
cm. In whn-t is the property of color inherent? Why does an object lying in green 
lu'ht look jrreen ? When does an object seen by ordinary light look green ? When 
does it look white ? When, black ? 670. What is it that determines what colors a 






COMPLEMENTARY COLORS. 257 

Changes of color are caused by changes of structure. We may show this 
by an experiment with a substance called iodide of mercury. This mineral 
is a bright scarlet ; when heated and allowed to cool undisturbed, it be- 
comes yellow ; but, the moment the surface is scratched, the particles re- 
arrange themselves, and the color turns back to scarlet. Here the same 
particles undergo a marked change of color by simply being made to assume 
a different arrangement. 

671. Complementary Colors. — Any two colors are 
said to be Complementary, when, if combined in due pro- 
portion, they will produce white. Those colors are com- 
plementary to each other which are distant half the length 
of the spectrum ; as, Red and green, 

Yellow and violet, 
Orange and blue. 

It is a curious fact that if we look intently at a bright object of any given 
color and then close our eyes, we shall still see it, but tinged with the com- 
plementary color. After gazing a few moments at a bright fire, everything 
we look at seems to have a greenish hue. If we place a red wafer on a piece 
of white paper and look at it intently, we shall soon see a circle of light green 
playing around it. A blue wafer will have a similar circle of orange, and a 
yellow wafer one of a violet tinge. 

672. A color appears to the best advantage, when placed 
beside its complementary color. 

Thus red is set off by green ; blue, by orange, &c. A pale face appears 
paler still when a black dress is worn. On white paper, black ink is plainer 
and pleasanter to the eye than ink of any other color. In arranging bou- 
quets, and selecting different articles of dress that are to be worn together, 
the effect of each individual color is heightened by bringing it in immediate 
contrast with its complementary color. 

673. Properties of the Spectrum. — Every ray of or- 
dinary sun-light appears to have three distinct properties : 
— 1. Brightness. 2. Heat. 3. Power of producing chem- 
ical effects. This last property is called Actinism. 

674. The chemical effects of sun-light are shown in various ways. Phos- 
phorus and nitrate of silver undergo a marked change when exposed to the 

substance absorbs, and what it reflects ? By what are changes of color caused ? 
Prove this with an experiment. 671. When are two colors said to be Complemen- 
tary? Name three pairs of complementary colors. What curious fact is stated with 
respect to complementary colors? Give examples. 672. When does a color appear 
to the best advantage? Give examples. 673. How many distiDct properties has 
*very ray of ordinary sun-light ? Name them, 674. Instance some of the chemical 

17 



258 optics. 

solar rays. Daguerreotypes and photographs are taken "by means of the 
action of light on sensitive chemical preparations. Almost all the colored 
vegetable juices, when exposed to sun-light, undergo a change of hue. Hy- 
drogen and chlorine, which may be mixed without danger in the dark, com- 
bine with a loud explosion in the light. Light, also, is essential to the 
chemical changes which result in the healthy growth of plants. Hence 
plants kept in a dark room become pale and sickly. A similar effect is 
produced on persons kept away from the light of the sun. 

675. Ordinary sun-light combines these three properties, 
but the seven colors into which it is decomposed do not 
possess them alike. Brightness belongs particularly to 
yellow ; heat, to red ; actinism, to violet and indigo. 

An object that is bright yellow makes a more vivid impression on the 
eye than one of any other color. Hence soldiers dressed in yellow are more 
distinct objects of aim to an enemy than those dressed in dark green or gray. 

The red portion of the spectrum has the most heat. This is shown by 
placing the bulb of a thermometer successively in each of the colors of the 
spectrum. It will be most affected by the red, but will show a still higher 
temperature, if brought a short distance below the red end of the spectrum, 
where no light falls at all. This shows that the heat of a solar ray is re- 
fracted as well as its light, but in a less degree. 

Actinism is strongest in violet and indigo rays. If a seed be placed un- 
der a dark-blue glass, so that all the light that strikes it will be tinged with 
that color, it will germinate in one-fourth of the time that it usually takes. 
Placed under a red glass, it will hardly germinate at all, because red, al- 
though it contains more heat than the other colors, has little or no actinism. 

676. Lines in the Spectktjm. — If solar light be ana- 
lyzed with an instrument called the Spectroscope, a great 
number of dark lines, parallel to each other but differing 
in breadth, will be seen crossing its surface. 

The position of these lines is always the same in the solar spectrum ; 
but, when a ray of star-light is decomposed, their number and arrange- 
ment are different, nor do they correspond in the case of different stars. 

The spectra of certain burning metals— iron, magnesium, sodium, zinc, 
copper, calcium, etc. — contain bright lines exactly corresponding to some 
of the dark lines in the spectra of the sun and stars. Now it is known that 



effects of sun-light. 675. Do the seven primary colors possess these three properties 
in equal degrees ? To which does "brightness particularly belong ? To which, heat ? 
To which, actinism ? What follows from the peculiar brightness of yellow ? How 
is it proved that the red portion of the spectrum has the most heat? How does the 
refraction of solar heat compare with that of solar light? Prove this. How is it 
shown that actinism is strongest in the blue rays ? 676. Describe the dark lines 
in the spectrum. What is said of the lines in the spectra of stars ? Whence is it 



ACHROMATIC LENSES. 259 

metallic vapors absorb the rays which they themselves emit ; hence it is 
inferred that the atmospheres of the sun and stars contain the vapors of 
the metals in question, and that these metals are therefore incandescent 
on the surface of those heavenly bodies. 

677. Dispersion of Light. — By the Dispersion of light 
is meant the formation of a spectrum from a single ray« 
Spectra formed by different refractive media are of differ- 
ent lengths. Thus flint-glass forms a spectrum about twice 
as long as crown-glass forms, and four times as long as wa- 
ter. Flint-glass is therefore said to have twice the disper- 
sive power of crown-glass, and four times that of water. 

678. Achromatic Lenses. — Lenses, like prisms, refract 
light, and produce spectra. Rays passing through a con- 
vex lens, therefore, instead of coming to a focus at a single 
point, are more or less dispersed, and form colored fringes 
about the focus. This defect is called Chromatic Aberra- 
tion. It was long a serious drawback in the use of optical 
instruments ; but the difficulty is now remedied by com- 
bining two lenses of such different materials that the dis- 
persive power of the one may nullify that of the other. 
Lenses combined on this principle are called Achromatic 
Lenses. 

Achromatic means colorless, and the lenses are so called because they do 
not fringe their images with the colors of the spectrum. A double convex 
lens of crown glass may be united with a plano-concave lens of flint glass. 
The latter corrects the chromatic aberration of the former, without entirely 
nullifying its converging effect. 

679. The Rainbow. — The Rainbow is an arch composed 
of the seven primary colors, which is visible in the sky 
when the sun shines during a shower. It appears in the 
opposite quarter to the sun, — in the west in the morning, 
and the east in the afternoon. 

When the sun is in the horizon, the rainbow is a circle ; but the lower 
part of it is intercepted by the earth's surface, and therefore we do not gen- 

inferred that certain metals are incandescent on the surface of the sun and stars? 
677. What is meant by the Dispersion of light ? When are media said to differ in dis- 
persive power ? 678. What is Chromatic Aberration ? How is it corrected ? What 
does achromatic mean ? Why are achromatic lenses so called ? How may an achro- 
matic lens be formed ? 679. What is the Rainbow ? Where is it seen ? What is the 



260 OPTICS. 

erally see more than a semi-circle. From the mast-head of a vessel or the 
top of a mountain, more than a semi-circle is visible. 

680. The rainbow is caused by the refraction and reflection of the sun's 
rays by drops of falling rain. Each drop operates like a prism, decomposing 
the light that strikes it. The observer's eye is so placed as to receive but 
one of the colors from one drop, but from other drops it receives the other 
colors, and thus has an arched spectrum formed complete. As no two per- 
sons occupy exactly the same spot, no two can see exactly the same bow. 

681. Sometimes two distinct bows are visible, one with- 
in the other. The inner one, which is called the Primary 
Bow, is the brighter of the two. The outer one is called 
the Secondary Bow; the rays that form it undergo one 
more reflection within the drop than those that form the 
primary bow, and are therefore fainter. In the primary 
bow, the arrangement of the colors is the same as in the 
solar spectrum ; in the secondary bow, this order is re- 
versed. 

682. Whenever the air is filled with drops, and the sun shines on them at 
a certain angle, rainbows are formed, which are visible to an observer in a 
proper position. Hence they are often seen in the spray of water-falls and 
fountains. 

683. Bows are sometimes similarly formed by moon-light, but they are 
faint and rarely seen. When so formed, they are called Lunar Rainbows. 

684. Halos. — Halos are luminous or colored circles 
seen around the sun and moon under certain conditions of 
the atmosphere. They are more frequently seen around 
the moon, because the sun's light is so intense that they 
are lost in its superior brightness. Halos arise from the 
refraction and dispersion of light by small crystals of ice 
floating in the higher regions of the atmosphere. 

Vision. 

685. The Eye. — The eye is the organ with which we 
see. Nothing more strikingly displays the wisdom of the 

form of the rainbow ? 680. Explain the principle on which the rainbow is formed. 
681. When two bows are formed, what is each called, and which is the brighter ? In 
what order are the colors arranged in the rainbow ? 682. By what besides rain may 
bows be produced? 683. What are Lunar Eainbows ? What is said of them? 
684. What are Halos ? Where are they most frequently seen ? How are halos pro' 



I 



THE EYE. 



261 



Creator than the nice adaptation of this wonderful instru- 
ment to the purposes for which it is designed. 

686. Parts of the Eye. — The human eye is a spheroid, 
about an inch in diameter, resting in a cavity below the 
forehead, capable of being moved upward, downward, or 
sidewise, by muscles attached to it behind. It consists of 
: an parts : — 



1. The Cornea. 

2. The Iris. 

3. The Pupil. 

4. The Aqueous Humor. 

5. The Crystalline Lens. 



6. The Vitreous Humor, 

7. The Ret'-i-na. 

8. The Choroid Coat. 

9. The Sclerotic Coat. 
10. The Optic Nerve. 




687. In an eye as set in the head (see Figure 253), 
some of these parts are hidden from view. A portion of 
the Sclerotic Coat {g g) appears ; and 
through the transparent Cornea, cov- 
ering the front of the globe and more 
convex than the rest of it, are seen the 
Iris (i i) and the Pupil (b). The 
Iris is the membrane, according to the color of which we 
say that the eye is blue or black, hazel or gray. The Pupil 
is a circular opening in the iris, through which light passes 
into the interior of the eye. Fig. 254 Fig. 254. 

represents a section of the eye. AAA 
is the cornea. 1 1 is the iris, and the 
opening in the centre is the pupil. In 
the following description reference is 
made to this Figure. 

On passing through the cornea, a ray of light 
enters the narrow apartment E, between the cor- 
nea on one side and the iris and crystalline lens on the other. This is filled 
.vith a transparent liquid resembling water, and called the Aqueous Humor. 
Traversing this, the ray next enters a transparent body, L, called from its 
shape the Crystalline Lens. Behind this is the Vitreous Humor, D, a trans- 




iueed ? 685, What is the eye ? 686. Describe the eye. Of how many parts does it 
consist ? Name them. 687. Which of these parts do we see when we look at an eye 
its set in the head ? What is the Cornea? What is the Iris? What is the Pupil ? 
With the aid of Fig. 254. name and describe the various parts of the eye. By what is 



262 OPTICS. 

parent fluid which fills the greater part of the globe of the eye. This humor 
is enclosed within the Retina, C C C, a delicate fibrous membrane resembling 
net-work, formed by the expansion of the optic nerve, on which every image 
seen by the eye is formed. The Optic Nerve, 0, passes through the back of 
the eye to the brain, and conveys to that organ the impressions made on the 
retina. 

The retina is surrounded by another coat called the Choroid, represented 
In the Figure by a dotted line. The choroid coat is lined on its inner surface 
with black coloring matter, to prevent any reflection of light from the inte- 
rior of the eye. Outside of all is the Sclerotic Coat, BBB, a strong mem- 
brane, to which the muscles that move the eye are attached. It envelopes the 
whole ball except the portion in front covered by the cornea, which fits into 
it just as the crystal of a watch fits into the case. 

688. Uses of the Different Parts. — The outer coats of 
the eye protect the delicate parts within. The cornea re- 
flects some of the light that falls on it, and this gives the 
eye its brilliancy. It transmits the greater part, however, 
and unites with the aqueous humor, the crystalline lens, and 
the vitreous humor, in bringing the incident rays to a focus 
and forming an image on the retina. 

The iris intuitively regulates the supply of light admit- 
ted into the eye, contracting and thus enlarging the pupil 
in a faint light, expanding and thus diminishing it in a 
strong one. These changes are not instantly made. Hence, 
when we pass from a bright light into a room partially 
darkened, we can hardly discern anything till the pupil en- 
larges, so that more rays are admitted. When we go from 
a dark room into a bright light,, the eye is pained, because 
the pupil, which had expanded to the utmost to accommo- 
date itself to the faint light, does not immediately contract, 
and more light is admitted than the sensitive membrane 
can endure. 

The pupils of cats, tigers, and animals generally that prowl at night for 
prey, are capable of being expanded to such a degree as to admit one hun- 
dred times as much light as when they are most contracted. They can there- 
fore see as well by night as by day. The owl's pupil is exceedingly large ; 



the retina surrounded ? With what is the choroid coat lined ? "What is outside ot 
all ? What are attached to the sclerotic coat ? 6S8. What is the use of the outer 
coats of the eye ? Of the cornea ? Which parts unite with the cornea in bringing 
•incident rays to a focus ? What is the use of the iris ? Give some familiar proofs that 
the iris accommodates itself to the intensity of the light. What is said of the pupil 



DEFECTS OF VISION. 263 

in the day-time, even when contracted to the utmost, it admits so much light 
that the bird is nearly blinded, and has to remain stupidly on its roost. 

689. Defects of Vision. — In a perfect eye, the rays 
that enter are brought to a focus on the retina, and an im- 
age is there formed. If the rays are not brought to a focus 
by the time they reach the retina, or come to a focus before 
reaching it, no impression is made on the optic nerve or 
communicated to the brain, and consequently no image 
is seen. 

Hence arise two defects of vision. When the cornea is 
too convex, distant objects form images in front of the ret- 
ina, and are not seen ; only such objects as are very near 
the eye are visible, and hence persons with this defect of 
vision are called near-sighted. When, on the contrary, the 
cornea is not convex enough, the rays are not brought to 
a focus by the time they reach the retina, and no image is 
seen. The eyes of old people generally labor under this 
defect, in consequence of the waste of a portion of the vit- 
reous and the aqueous humor, so that the crystalline lens 
and the cornea fall in. This falling in is just what the near- 
sighted person needs ; accordingly it is often found that 
those who are near-sighted in youth see perfectly well when 
they grow old. 

690. The two defects of vision mentioned above are remedied by the use 
of spectacles, which consist of lenses of different shapes placed in frames be- 
fore the eyes. A near-sighted person uses glasses just concave enough to 
nullify the too great convexity of his eye. An old person uses glasses with 
sufficient convexity to make up the deficiency of his eye in that respect. 

691. Spectacles were first used about the end of the thirteenth century. 
It is supposed that the world is indebted to Roger Bacon for their invention. 
Before that time all near-sighted and most aged persons had to remain in a 
state of comparative blindness. 

692. Though all other parts of the eye be perfect, if the optic nerve does 
not perform its functions, blindness is the result. Images are formed on the 
retina, but there is no communication with the brain, and no impression 

of beasts that prowl at night? What is said of the owl's pupil ? 689. Where are im- 
ages formed in a perfect eye ? What will prevent an image from being seen ? De- 
scribe the two defects of vision arising from images' not being formed on the retina. 
690. How are these two defects of vision remedied ? What sort of glasses does a near- 
sighted person use ? An old person ? 691. When were spectacles first used ? By 
whom are they supposed to have been invented ? 692. If the optic nerve docs not 



Zt54 



OPTICS. 



is produced. For amaurosis, or paralysis of the optic nerve, there is no 
remedy. 

693. Images fokmed on the Retina. — Images are 
formed on the retina, just as in a dark room, by light ad- 
mitted through an aperture (see Fig. 235). In the latter 
case, as we have already seen, the image is inverted, and it 
follows that images formed on the retina must be inverted 
also. Why then do we see them in their natural position? 
This question it is hard to answer. The explanation com- 
monly given is this : — That we see all things inverted, and 
have always done so ; but, inasmuch as we know by expe- 
rience that they are erect, the mind of itself, insensibly to 
us, corrects the delusion that the inversion would other- 
wise produce. We have no means of comparison ; we see 
nothing erect, to serve as a standard and prove the general 
inversion. 

694. Another question is sometimes asked : — Since we have two eyes, and 
two images are formed, one on each retina, why do we not see two images of 
every object ? The answer is, because both eyes are inclined to any given 
object at nearly the same angle. The images produced on the retinas are very 
nearly the same. The impressions transmitted to the brain by the two branches 
of the optic nerve are identical and simultaneous, and but one perception is 
the result. If we press on one of our eyes, so as to incline it towards an ob- 
ject at a different angle from the other, we see two images. Drunken men 
often see double, because they lose control of the muscles of the eye, and do 
not direct both eyes towards a given object at the same angle. 

695. Visual Angle. — The visual angle is the angle 
formed by two lines drawn from the eye to the extremities 

of a given object. 
In Fig. 255, the vis- 
ual angle of the ar- 
rowBA is BEA; 
that of the arrow 
CDisCED. 
A given object 




perform Its functions, what is the consequence ? 693. What kind of an image is 
termed on the retina, and why? Since an inverted image is formed on the retina, 
why do we see objects in an erect position? 694. Since we hare two eyes, why do 
we not see two images of every object? How may we make two images visible ? 
Why do drunken men often see double 9 695. What is the Visual Angle ? Show tho 



THE VISUAL, AJMGLE. 265 

looks large or small, according to the visual angle that it 
forms. Two equal arrows held up before the eye at differ- 
ent distances, as in Fig. 255, form different visual angles, 
and therefore seem to be of different size. If we measure 
their apparent lengths with an interposed rod, we shall find 
the nearer one to measure the distance a b, the farther one 
only about half as much, c d. A small object placed near 
;he eye may form as great a visual angle as a very large 
distant object, and may therefore entirely hide the latter 
when interposed between it and the eye. 

Accordingly, the nearer an object is brought to the eye, the larger it ap- 
pears to be, and tbe farther it is removed the smaller it looks. When the 
visual angle is less than V300 of a degree, an object becomes invisible. A 
bird flying from us grows smaller and smaller, till its visual angle dimin- 
ishes so that it can no longer be seen, and we say that it has gone out of sight. 

696. In the case of familiar objects, experience prevents us from being 
misled by their apparent size. Insensibly to ourselves, we make allowance 
for their distance, of which we judge by the distinctness of their outline and 
by intervening objects. A man at work on a lofty steeple may not look more 
than two feet high, yet we are in no danger of mistaking him for a dwarf. A 
distant tree seems to be no higher than a bush ; but, if we see a horse feed- 
ing beneath it, we intuitively compare the two, and arrive at a correct idea 
of the tree's size. 

A white object can be distinguished at a greater distance than one of any 
other color, and is visible twice as far when the sun shines directly on it as 
when simply illumined by ordinary light. An object is brought out most 
distinctly by a back-ground which contrasts strikingly with it in color. 
Dark-colored eyes, for the most part, see farther than light ones ; and those 
who are in the habit of looking at remote objects, like sailors, can discern 
piinute bodies at distances which render them invisible to ordinary sight. 

697. Adaptation of the Eye. — One of the most re- 
markable properties of the eye is its power of adapting 
itself to different intensities of light and different dis- 
tances. The pupil, by expanding and contracting, regu- 
lates in a measure the supply of light ; still, the difference 
3f intensity in the light admitted to the eye under different 

/isual angles of the arrows in Fig. 255. On what does the apparent size of an object 
depend? Illustrate this with the Figure. When does an object become invisible? 
When is a bird said to go out of sight ? 696. In che case of familiar objects, what pre- 
vents us from being misled as to their size ? Give some familiar examples. What 
.•olur must an object be, to be distinguished at the greatest distance ? How is an ob- 
ject most distinctly brought out? What is said of da-k-colored eyes? 697 What is 



266 optics. 

circumstances is very great. We can read by the light of 
the moon and by that of the sun ; yet the latter is 547,500 
times as intense as the former. 

698. Again, the eye adapts itself to different distances. 
If we look at a remote object through a telescope, we have 
to pull out the tube to a certain length, according to the 
distance, before we can see it to advantage. No such arti- 
ficial adjustment is necessary with the eye. We look suc- 
cessively at objects 1, 5, 10, and 20 feet off; and in each 
case the eye instantly adapts itself to the distance. 

699. An object may move with such velocity that we 
can not see it, as is the case with a cannon-ball. This is 
because the image formed on the retina does not remain 
sufficiently long to produce an impression. When an image 
is once formed, it remains from one-sixth to one-third of a 
second after the object has disappeared. Hence a burning 
stick whirled rapidly round seems to form a circle of fire, 
and a meteor or a flash of lightning produces a continuous 
train of light in the heavens. 

Optical Instruments. 

700. The Stereoscope. — The Stereoscope is a combi- 
nation of two double- convex lenses, so placed, one in front 
of each eye, as to form from two pictures of any solid ob- 
ject or scene, a single image which appears to stand out 
in relief. The two pictures are photographs (p. 268), taken 
from two slightly different points, so as to show the object 
or scene just as it would appear to each eye separately. 

701. The Camera Obscura. — When rays from an ob- 
ject brilliantly illuminated are admitted through an aper- 
ture into a dark room, an inverted but indistinct image is 
formed. We may give it a sharper outline by placing a 
double-convex lens in the aperture, and receiving the image 

one of the most remarkable properties of the eye ? Give an example of the differ- 
ence of intensity in the light admitted to the eye. 698. Show how the eye adapts 
itself to different distances. 699. Why is it that an object moving with very great 
velocity is not seen ? When an image is once formed, how Ions does it remain after 
the object has disappeared ? Give examples. 700. What is the Stereoscope ? What 
ire stereoscopic views ? 701. What is meant by the Camera Obscura ? How is 






THE CAMERA OBSCURA. 



267 



on a white ground at its focus. Such an arrangement is 
called the Camera Obscura, or dark chamber. 

For practical purposes, the camera obscura must be 
portable. A close box, painted black on the inside, is 
therefore substituted for the darkened room. This instru- 
ment enables the draughtsman to sketch material objects 
or natural scenery with great ease and accuracy, and is in- 
dispensable to the daguerreotypist and photographer. 



Fig. 256. 




DRAUGHTSMAN 



702. Draughtsman's Camera. — Fig. 256 
represents the camera as used by draughts- 
men. To be conveniently traced, the image 
must be thrown pn a horizontal surface, and 
this is effected by making the opening in the 
top of the box and receiving the rays on a 
mirror, A, inclined at an angle of forty-five 
degrees. From this mirror they are reflect- 
ed to a meniscus, B, which crosses the aper- 
ture, and are by it refracted to the horizontal 
surface, C D, where, on. white paper placed to 
receive it, is formed a distinct image, which 
can be readily traced with a pencil. The up- 
per part of the draughtsman's person is ad- 
mitted through an opening in the side of the 
box, over which a dark curtain must be 
drawn, so as to exclude all light except what enters from above. 

Fig. 257. 703. Photographer's Camera. — Asusedintak- 

daguerreotypes and photographs, the camera 
the form shown in Fig. 257. A is a brass 
containing a corn- 
achromatic lenses, 
adjusting screw 
is moved out far 
enough to bring 
the focus at the 
right spot. The 
image is received 
on a piece of 
ground glass, fit- 
ted into a frame, 
which slides in 
a groove in the 
back of the cam- 



bination 
which 




photographer's camera. 



'the .camera made portable ? By whom is the camera used ? 702. Describe the 
draughtsman's camera. 703. Describe the photographer's camera. How is the plat* 



268 optics. 

era. When a daguerreotype is to be taken, the ground glass is withdrawn, 
and another frame, C, containing a prepared plate, carefully shielded from 
the light, is introduced in its place. A door in front of C is then raised, 
and the image formed by the lenses is thus allowed to fall on the plate. 

Photographic Process. — In the daguerreotype process, the plate is of 
copper thinly covered on one side with silver, which is rendered sensitive 
by exposure to the vapor of iodine. In making a negative from which to print 
photographs, the plate used is of glass, coated with collodion (a solution 
of gun-cotton in ether and alcohol, impregnated with an iodide), and then 
immersed in a nitrate-of-silver bath. The rays transmitted through the 
camera, by that property inherent in them which we have called actinism, 
in a few seconds produce a chemical effect on the sensitive surface, and the 
plate is then removed to a dark room. No change is visible on its sur- 
face ; but, as soon as a solution of sulphate of iron mixed with acetic acid is 
poured on it, the picture begins to appear, and soon becomes distinct. The 
image thus developed is fixed by immersing the plate in a solution of 
cyanide of potassium. It is then washed in water, dried- and varnished. 

"When it is desired to print photographs from a negative albumen-paper 
is floated on a bath of nitrate of silver, dried, and put in a printing-frame. 
The coated surface of the negative is then pressed down on the sensitive 
surface of the paper, and in this position exposed to the sun. The impres- 
sion obtained is then washed in two or three waters, and toned in a solution 
of chloride of gold. It is then fixed by immersing it in a solution of hy- 
posulphite of soda, thoroughly washed, and dried. 

704. The Miceoscope is an instrument which enables 
us to see objects too small to be discerned by the naked 
eye. This is the case with objects whose visual angle is 
less than -^ of one degree ; the microscope enables us to 
see them by increasing their visual angle. 

Microscopes are either Simple or Compound. A Simple 
Microscope is one through which the object is viewed 
directly. With the Compound Microscope a magnified 
image of the object is viewed, and not the object itself. 

105. The Simple Microscope. — The simple microscope 
consists of a double-convex lens (or sometimes more than 
one), which operates on the principle shown in Fig. 258. 

The arrow h c would be seen by the naked eye under the visual angle 
b A c. When the lens m is interposed, the rays are so refracted as to form 

prepared in the daguerreotype process ? How, in the photographic process ? How 
are photographs printed from a negative ? 704. What is the Microscope ? What is a 
Simple Microscope? What is a Compound Microscope? 705. Of what does the 
simple microscope consist ? With Figure 258, explain the principle on which the 



THE MICROSCOPE. 



269 




the visual angle DAE, and the arrow 7) Fig. 258. 

appears to be of the size D E, much 
larger than it really is. Sometimes 
an exceedingly minute object becomes 
visible when brought very near the 
eye, but in that position the rays en- 
ter the eye with such divergency that 
a confused image is produced. The 
microscope corrects this excessive divergency, and presents a clear and mag- 
nified image. 

706. The Compound Microscope. — The compound mi- 
croscope is a combination of two, three, or four convex 
lenses, through which we view a magnified image of an 
object instead of the object itself. The lenses are fixed in 
tubes moving one within the other, and suitable apparatus 
is provided for adjusting them, for holding the object un- 
der examination, and throwing on it a strong light. When 
but two lenses are employed, they are arranged as repre- 
sented in Fig. 259. 

Fig. 259. 




FY;'- 



D E is the object, and B, the lens nearest to it, is called the object-glass. 
C, the lens nearest the eye, is called the eye-glass. A magnified image of the 
arrow is formed at H I by the lens B. This image is viewed through the 
lens C, and is thus still further magnified, being seen under an increased 
visual angle at F G. If the magnifying power of B is 20, and that of C 4, the 
image seen will be 80 times the size of life. 

707. Solar and Oxy-hydrogen Microscopes. — These mi- 
croscopes are used for throwing magnified images on a 
white screen in a darkened room. 



simple microscope operates. 706. Describe the compound microscope. With the aid 
of Fig. 259, name the parts and show the operation of the compound microscope. 
T07. For what are the Solar and the Oxy-hydrogen Microscope used ? Describe the 



270 OPTICS. 

In the case of the Solar Microscope, an aperture is made 
in one of the shutters. Outside of this a mirror is placed, 
in the sun, at such an angle as to reflect the rays that fall 
on it through a horizontal tube towards the object to be 
magnified. They first fall on a convex lens, and then on a 
second, which brings them to a focus on the object, and 
thus illuminates it brilliantly. Another lens, at the oppo- 
site extremity of the instrument, produces the magnifying 
effect. A screen, from ten to twenty feet off", receives the 
image, which increases in size with the distance. If the 
screen is too far removed, the image becomes faint ; but 
so powerful is the light concentrated on the object that a 
very great magnifying effect may be produced without any 
lack of distinctness. 

In the Oxy-hydrogen Microscope, the principle is the same, but the bril- 
liant light produced by burning lime in a current of oxygen and hydrogen is 
substituted for the rays of the sun. Accordingly, with this instrument, the 
aperture in the shutter and the mirror on the outside are unnecessary. Fig. 
260 shows the operation of the oxy-hydrogen microscope. 

Fig. 260. B represents an intense 

white light produced by the 
burning of a cylinder of lime 
in a current of oxygen and hy- 
drogen combined. This light 
falls on the reflector A, by 
which it is thrown back on the double convex lens C, and this brings it to a 
focus on the object D. E is an achromatic Jens, which throws a magnified 
image on the screen. 

708. The microscope introduces us to new worlds, of the very existence 
of which we would otherwise have been ignorant. It reveals to us, in every 
drop of water in which vegetable matter has been infused, swarming myriads 
of moving creatures, — miniature eels, infinitesimal lobsters, ravenous mon° 
sters with distended jaws preying on their feebler fellows, — all endowed with 
the organs of life, and so minute that their little drop is to them a world nearly 
as large as ours to us. It shows us the feeding apparatus of the flea magni- 
fied to frightful dimensions, and his body arrayed in a panoply of shining 
and curiously jointed scales, studded at intervals with long spikes. The 
mould on decaying fruit it magnifies into bushes with branches and leaves, 



6olar microscope, and its operation. What is the effect of removing the screen to a 
greater distance from the instrument ? What light is employed in the oxy-hydrogen 
microscope ? With Fig. 260, show how this microscope operates. 708. What is said 
of the revelations of the microscope ? What difference does it exhibit between tho 




THE MAGIC LANTERN. 



271 



displaying all the regularity and beauty of the vegetable creation. It dis- 
closes to us many striking facts connected with physiology and chemistry. 
It shows us the imperfection of the finest works of art, when compared with 
those of nature. The edge of the sharpest razor, viewed through a micro- 
scope, is full of notches ; the point of a needle is blunt, and its surface is cov- 
ered with inequalities. The magnified sting of a bee, on the other hand, is 
perfectly smooth, regular, and pointed. The finest thread of cotton, linen, 
or silk, is rough and jagged : whereas in the filament of a spider's web not the 
slightest irregularity can be detected. — In a word, the revelations of the mi- 
croscope are in the highest degree wonderful and interesting ; and, to what- 
ever we direct it, we always find abundant matter to reward our labor and 
stimulate us to further researches. 

709. The Magic Lanteen. — The Magic Lantern is an 
instrument for throwing on a screen magnified images of 
transparent objects. It operates on the same principle as 
the oxy-hydrogen microscope, but for its illuminating power 
has an ordinary lamp instead of the intense light produced 
by burning lime. 

Fig. 261. 




THE MAGIC LANTERN. 



Fig. 261 represents the magic lantern. L is the lamp. M N is the re- 
flector, which throws the light on the lens A. This lens brings it to a focus 
on the picture, which is painted on a glass slider and introduced into the 
opening C D. The lens B receives the rays from the slider, and throws a 
magnified image on the screen F. 

710. Phantasmagoria. — When a powerful light is used, 
and the tube containing the magnifying lens or lenses is 
capable of being drawn out or pushed in, so as to bring 
them at different distances from the object, we have what is 
called a Phantasmagoria Lantern. 



works of art and those of nature ? 709. What is the Magic Lantern ? How does it 
•liffer from the oxy-hydrogen microscope? With Fig. 261, describe the magic Ian- 



272 optics. 

To exhibit the Phantasmagoria, a transparent screen is suspended, on one 
side of which is the exhibitor with his lantern, on the other the spectators. 
Having brought the lantern close to the screen and drawn out the tube till 
the image (which will be quite small) is perfect, the exhibitor walks slowly- 
back. He thus gradually increases the size of the image, while he preserves 
its distinctness by pushing in the tube as he recedes. The effect on the 
spectators is startling. The room being dark, they can not see the screen, 
but only the illuminated image, which, as it grows larger, appears to be 
moving towards them ; even those who are familiar with the instrument can 
hardly disabuse their minds of this impression. When the exhibitor ap- 
proaches the screen and pulls out the tube, the image becomes smaller and 
appears to recede. 

711. Dissolving Views. — Dissolving Views, in which 
one picture appears to melt into another, are produced by 
two magic lanterns, inclined so as to throw their images on 
the same spot. An opaque shade is made to revolve in 
front of the instruments, in such a way as gradually to in- 
tercept the rays from one and uncover the tube of the other. 
The first picture fades, and a new one takes its place, be- 
coming more and more distinct as the other disappears. 

712. The Telescope. — The Telescope is an instrument 
for viewing distant objects. It appears to have been in- 
vented by Metius, a native of Holland, in 1608. The fol- 
lowing year, Galileo, hearing of the new instrument, con- 
structed one for himself, and was the first to make a 
practical use of the invention. To the Telescope, Astron- 
omy is indebted for the important advances it has made 
during the last two centuries. 

Telescopes are of two kinds, Refracting and Reflecting. 
In the former, which were the first constructed, lenses are 
used ; in the latter, polished metallic mirrors. 

713. Refracting Telescopes. — The simplest form of the 
telescope is that devised by Galileo. It is a tube contain- 
ing a convex object-glass and a concave eye-glass. By the 
former parallel pencils are made to converge towards a 
focus, where they would form an inverted image ; but be- 
te™. 710. What is the Phantasmagoria Lantern ? How are the phantasmagoria pro- 
duced? What is said of their effect ? 711. What are Dissolving Views ? How are 
they produced ? 712. What is the Telescope? By whom was it invented? Who 
first made a practical use of the invention? Name the two kinds of telescopes. 



THE TELESCOPE. 



273 



fore reaching the focus they fall on the concave lens, and 
have their convergency so far corrected that an object is 
distinctly seen by an eye at the extremity of the tube. The 
Opera-glass consists of two Galilean Telescopes combined. 
The night-glass used by sailors is on the same plan. 

In the instrument called the Astronomical Telescope, both object-glass and 
eye-glass are convex. The former produces an inverted image at its focus ; 
the latter, which is so placed that its focus falls at the same spot, refracts the 
rays diverging from this image, and thus renders it visible to the eye. The 
inversion of the image is of no consequence in observing the heavenly bodies ; 
but, when objects on the earth are viewed, we want an erect image, and there- 
fore in the Terrestrial Telescope two additional lenses are introduced to cor- 
rect the inversion. 

714. Reflecting Telescopes. — In Reflecting Telescopes, 
a speculum, or mirror, takes the place of the object-glass. 
These instruments appear in several different forms. The 
principle on which Herschel's is constructed, will be under- 
stood from Fig. 262. 



Fig. 262. 




The mirror SS is 
placed at the farthest 
extremity of the tube, 
inclined so as to make 
the rays that fall upon 
it converge towards the 
side of the tube in which 
the eye-piece a b is fixed to receive them. The observer at E, with his back 
towards the heavenly body, looks through the eye-piece, and sees the reflect- 
ed image. His position is such as not to prevent the rays from entering the 
open end of the tube. The advantage gained with this instrument depends 
in a great measure on the size of the mirror ; for all the rays that fall on it 
are concentrated and transmitted to the eye. 

715. The largest telescope ever constructed was made by the Earl of Rosse. 
The great mirror is six feet in diameter, and weighs four tons. The tube, at 
the bottom of which it is placed, is of wood hooped with iron. It is fifty-two 
feet long and seven feet across. It is computed that with this instrument 
250,000 times as much light from a heavenly body is collected and transmit- 
ted to the eye as ordinarily reaches it. 



713. Describe the Galilean Telescope. Of what does the Opera-glass consist ? De- 
scribe the Astronomical Telescope. How does the Terrestrial Telescope differ from 
the Astronomical ? 714. In reflecting telescopes, what takes the place of the object- 
glass ? With Fig. 262, explain the principle on which Herschefs Telescope operates. 
On what does the advantage gained with this instrument depend ? 715. Describe the 
telescope of the Earl of Eosse. How great i» the advantage gained with it ? 

18 



274 optics. 



EXAMPLES FOR PRACTICE. 

1. (See § 594.) How long does it take a ray from the moon to reach the earth, 

the moon's distance being 240,000 miles? 

2. The planet Jupiter is 476,000,000 miles from the sun. How long does it 

take a ray of light from the sun to reach the planet ? 
5, A ray of light from the sun is about 12,273 seconds longer in reaching the 
newly discovered planet Neptune than in reaching Jupiter. About how 
many miles farther from the sun is Neptune than Jupiter ? 

4. (See § 595. ) A holds his book 1 foot, and B holds his 3 feet, from a certain 

candle. How much more light does A receive than B ? 

5. The planet Uranus is twice as far from the sun as the planet Saturn. 

How does the light received at Saturn compare in intensity with that re 
ceiled at Uranus ? 

6. (See § 650.) How many times is the ordinary heat of the sun increased by 

a burning glass with an area of 10 square inches, the focus of which has 
an area of Vio of a square inch ? 

7. A convex lens has a focus 1 / 5 of a square inch in area, and increases the 

heat of ordinary sun-light 200 times ; what is the area of the lens? 



CHAPTER XV, 



ACOUSTICS, 



716. Acoustics is the science that treats of sound. 

71 7o Nature and Origin of Sound. — Sound is an im- 
pression made on the organs of hearing by the vibrations 
of elastic bodies, transmitted through the air or some other 
medium. These vibrations may be compared to the mi- 
nute waves which ripple the surface of a pond when a stone 
is thrown in, — spreading out from a centre, but growing 
smaller and smaller as they recede, till finally they are no 
longer perceptible. They are produced by percussion, or 
any shock which puts in vibration the molecules of the 
sounding body. There is no sound that can not be traced 
to mechanical action. 

718. Bodies whose vibrations produce clear and regular 



SOUND PRODUCED BY VIBRATIONS. 275 

sounds are called Sonorous. Bell-metal, glass, the head of 
a drum, are sonorous. 

719. That sound is produced by vibrations is proved in various ways. A 
person standing near a piano-forte or an organ, when it is played, feels a 
tremulous motion in the floor of the apartment, as well as in the instru- 
ment itself if he touches it. We perceive the same tremor in a bell when 
m the act of being rung. In like manner, if we strike a tumbler so as to pro- 
duce a sound, and then touch the top, we feel an internal agitation ; and, 
when the vibrations are stopped, as they are by contact with the finger, tha 
sound ceases with them. If we put water in a glass and produce a sound b_y 
rubbing the top with the finger, the liquid is agitated, and its motion contin- 
ues until the sound dies away. — Place some fine sand on a square piece of 
glass, and, holding it firmly with a pair of pincers, draw a violin-bow along 
the edge. The sand is put in motion, and finally settles on those parts of 
the glass that have the least vibratory movement. — If a tuning-fork be struck 
and applied to the surface of mercury, minute undulations may be observed 
in the metal. 

That these vibrations are communicated to the air and by it transmitted 
to the ear, also admits of easy proof. The rapid passage of a heavy cart or 
stage shakes the walls of a house. The discharge of artillery sometimes breaks 
windows. These effects are due to the vibrations suddenly produced in the 
air. If there is no air or other medium to transmit the vibrations to the ear, 
no sound is heard. We have already seen (§ 439) that a bell rung in an ex- 
hausted receiver can hardly be heard ; if the air could be entirely removed, 
it would be wholly inaudible. Sound, therefore, does not leap from point to 
point, but is transmitted by vibrations communicated from one particle to 
another. 

720. All sonorous bodies are elastic, but all elastic bodies 
are not sonorous. 

Soft bodies are generally non-elastic, and consequently not sonorous. 
This is the case with cotton, for example, which yields little or no sound 
when struck by a hammer. It is on this account that music loses much of its 
effect in rooms with tapestried walls or curtained windows. Hence, also, a 
speaker finds it more difficult to make himself heard in a crowded room than 
'■n one that is empty. 

721. Transmission of Sound. — All the sounds that or 



716. What is Acoustics ? 717. What is Sound ? How are sound-waves produced ? 
To what is every sound traceable ? 718. What bodies are called Sonorous ? Give 
2xamples. 719. How is it proved by familiar experiments that sound is produced by 
vibrations ? If a tuning-fork bo struck and applied to the surface of mercury, what 
may be observed ? How is it proved that these vibrations are communicated to the 
air and by it transmitted to the ear ? 720. What property belongs to all sonorous 
bodies ? What bodies are, for the most part, not sonorous ? Give examples. What 
'ollows from the fact that soft bodies are not sonorous ? 721. By what are the sounds 



276 ACOUSTICS. 

dinarily reach our ears are transmitted to them by the air. 
Any material substance, however, that connects our organs 
of hearing with a vibrating body, may transmit the vibra- 
tions in the same way. Thus, with our heads immersed in 
water, we can hear a sound produced under the surface at 
a considerable distance. Here water is the transmitting 
medium. 

722. Liquids are better conductors of sound than aeri- 
form bodies, and solids than liquids. 

Persons in boats can converse with each other at a great distance, be- 
cause water is a good conductor of sound. When the ear is applied to one 
end of a long stick of timber, the scratch of a pin at the other end can be 
distinctly heard, owing to the conducting power of the wood. An approaching 
locomotive can be heard at a great distance by placing one's ear on the rails. 
The American Indians knew by experience the facility with which solids 
transmit sounds, and were in the habit of applying their ears to the earth 
when they suspected the approach of an enemy, or wanted a more distinci 
impression of any sound that attracted their attention. 

723. The denser air is, the more readily it transmits sounds. On the 
tops of high mountains, where, as we have already learned, the atmosphere 
is rare, the human voice can be heard only a few rods off, and the report of 
a musket sounds no louder than the snapping of a whip at the level of the 
sea. On the other hand, the air in a diving-bell let down to the bottom of 
the sea, which is condensed by the upward pressure of the water, transmits 
sound so freely that those who descend can hardly speak to each other above 
their breath ; conversation in an ordinary tone would pain the ear.— Frosty 
air is a much better conductor of sound than warm air. In the polar re- 
gions, conversation has been carried on by two persons a mile apart. 

Still air of uniform density transmits sounds more freely than air which 
is agitated by variable currents or contains strata of different density. This 
is one reason why sounds are more distinctly heard by night than by day. 
Falling rain or snow interferes with the vibrations, and tends to make 
counds less distinct ; so, likewise, do contrary winds. 

724. If the air were perfectly still and of uniform densi. 
ty, sound transmitted through it would decrease in loud* 
ness as the square of the distance from the vibrating body 

we ordinarily hear, transmitted ? What else may transmit sound-waves in the same 
way ? 722. How do solid, liquid, and aeriform bodies compare, as conductors of 
sound ? Give a proof of the conducting power of water. State some facts illustrating 
the facility with which solids conduct sound. 723. How do rare and dense air com- 
pare, as conductors of sound ? Give examples. How does cold air compare with warm 
In conducting power ? Under what circumstances does air transmit sound most free* 
i} ? What is the effect of falling rain or snow? 724. If the air were perfectly still 






TELOCITY OF SOUND. 277 

increased. The report of a cannon, for instance, would 
seem only one-fourth as loud at a distance of 200 feet as 
at a distance of 1 00 feet. 

725. Velocity of Sound. — Under ordinary circum- 
stances, at a temperature of 60° F., sound is transmitted 
through air with a velocity of 1,120 feet in a second, which 
is at the rate of a mile in about 4f seconds. 

All sounds, whether loud or faint, high or low, are 
transmitted by a given medium with very nearly equal 
rapidity. Were it not so, there would be no such thing as 
harmony in musical performances, for the notes of the dif- 
ferent instruments would reach the ear at different intervals. 

Sound, it will be observed, travels much more slowly than light. The 
latter moves 185,000 miles while the former is going only 1,120 feet. The 
difference in their velocities is perceptible even at short distances. If we 
look at a man splitting wood a few rods off, we see the axe descend on the 
log some time before we hear the noise of the blow. So, the report of a can- 
non is not heard till after the flash is seen, — the interval being long or short 
according to its distance. 

726. When the sound is accompanied with a flash, knowing the relative 
velocity of sound and light, we can calculate very nearly the distance from 
which it comes. We have only to notice the number of seconds that elapse 
after the flash is seen before the sound is heard, and multiplying this by 
1,120, we get the distance in feet. The time which it takes the light to trav- 
erse the given distance and reach the eye, is so small that it does not enter 
into the calculation. For example, if a clap of thunder is heard 3 seconds 
after the accompanying flash is seen, the cloud from which they proceed is 3 
times 1,120 (or 3,300) feet distant. The sooner the report follows the flash, 
the nearer the cloud. 

727. Water transmits sound 4^ times as rapidly as air ; 
iron, 10 times; and different kinds of wood, from 11 to 17 
times. 

Place the ear at one end of a very long stick of timber, and let some one 
strike the other end with a hammer. The wood conducts the sound to the 
ear so much more quickly than the air that the blow is heard twice. So, 

and of uniform density, what would be the law for the loudness of a sound heard at 
different distances? Give an example. 725. What is the velocity of sound? How 
is the velocity of sound affected by its loudness and pitch? What proof have we of 
this? How does the velocity of sound compare with that of light? Give some fa- 
miliar instances showing their difference of velocity. 726. When the sound is accom- 
panied with a flash, how may we calculate the distance from which it comes ? Give 
an example. 727. With what velocity does water transmit sound, as compared with. 



278 ACOUSTICS. 

when a bell at the end of a long iron tube is struck, two sounds are heard at 
the opposite extremity, — the first conducted by the iron, the second by the 
air within it. 

728. Distance to which Sound is transmitted. — So 
many changes are constantly taking place in the atmos- 
phere, in its temperature, moisture, density, and the veloc- 
ity and direction of its currents, that no universal law can 
be laid down as to the distance at which sound is audible. 
The human voice, when raised to its highest pitch and loud- 
est tones, may be heard at the distance of an eighth of a 
mile ; the report of a musket, at 5 miles. 

Through the water, or in the atmosphere directly over it, sounds are trans- 
mitted to a great distance. The ringing of a bell under water has been heard 
across the whole breadth of Lake Geneva, not less than nine miles. The 
" all's well " of the sentinel at Gibraltar has been distinguished twelve miles 
off, and naval engagements have been heard at a distance of 200 miles. An 
eruption of the volcano of St. Vincent has been heard at Demerara, 340 miles 
off, — the greatest distance on record to which sound has been transmitted by 
the atmosphere. 

729. Acoustic Tubes. — It is their dispersion in the sur- 
rounding air that makes sounds finally inaudible. Hence, 
when they are confined within tubes, they are carried to a 
much greater distance. The slightest whisper has been 
heard through an iron pipe 3,120 feet (more than half a 
mile) in length. 

This fact has been turned to account in several ways. The voice is con- 
veyed by speaking-tubes from one part of a building to another, frequently 
to a considerable distance and by a circuitous route. The Stethoscope, an 
instrument for examining the lungs and other internal organs, is an applica- 
tion of the same principle. It is a hollow cylinder of wood with a funnel- 
shaped extremity, which is placed on the organ to be examined while the ear 
is applied to the other end. The sounds produced by the vital action within 
are thus conveyed to the ear, and enable the experienced examiner to judge 
whether the organ is in a healthy state. 



air ? Iron ? Wood ? What experiments prove that solids conduct sound more rap- 
idly than air ? 728. What makes it impossible to lay down a universal law as to the 
distance at which sound is audible ? How far may the human voice be heard? The 
report of a musket ? What instances are mentioned showing the great distance to 
which sound is transmitted by water ? What is the greatest distance on record to 
which sound has been transmitted by the atmosphere ? 729, What makes sounds 
finally inaudible ? How may this difficulty be in a measure removed ? How far has 
ft feint whisper been heard through a tube ? How has this principle been turned to 



THE SPEAKING-TRUMPET. 279 

730. The Speaking-trumpet. — Even if the tube is short, 
the more intense pulsation excited in a column of confined 
air makes a given sound audible at a much greater distance 
than if it is at once diffused in the atmosphere. This is 
proved by the Speaking-trumpet, an instrument used by 
seamen and others who wish to give additional power to 
their voices. The narrowness of the tube prevents the easy 
flow of the air which the voice sets in vibration. The or- 
gans of articulation, therefore, operate on it with concen- 
trated force, as they do on condensed air ; and, conse- 
quently, when the vibrations escape from the tube, they 
are propelled to a greater distance. A loud voice with a 
speaking-trumpet 20 feet long, can be heard at a distance 
of three miles. No one can use the speaking-trumpet long 
without being exhausted, which shows that an unusual 
effort has to be made with the voice. 

731. Interference of Sound. — Two sets of vibrations 
of equal intensity, meeting in such a way that the depres- 
sions of one correspond with the elevations of the other, 
interfere, or neutralize each other, and an interval of silence 
is the result. 

Cause a tuning-fork to vibrate and hold it over a cylindrical glass vessel. 
Vibrations will soon be communicated to the glass, and a musical note will 
be heard. Place a similar glass vessel at right angles to the first and oppo- 
site the tuning-fork, and the note previously heard will cease. Withdraw it, 
and the note is again heard. The vibrations of the first vessel produce the 
sound, but are neutralized by those of the second. 

732. Reflection of Sound. — Vibrations striking a 
plane surface are reflected from it (like light and heat) in 
such a way as to make the angle of reflection equal to the 
angle of incidence. 

733. Echoes. — When a sound is heard a second time by 
reflection, after a certain interval, an Echo is said to be 
produced. A sound is sometimes repeated more than once, 

account? What instrument is constructed on this principle ? Describe the Stetho- 
scope, and its operation. 730. By whom is the Speaking-trumpet used ? Explain the 
principle on which it operates. How far has a loud voice been heard with a speaking- 
trumpet? 731. What is meant by the Interference of sound, and how is it caused ? 
Give »n example. 732. What is the law for the reflection of sound ? 733. What is an 



280 acoustics. 

according to the number of reflecting surfaces on which it 
strikes. An echo near Milan repeats a single syllable thirty 
times. 

To be distinctly beard, tbe echo must not reach the ear till one-ninth of a 
second after the original sound has ceased. Otherwise they will run together 
and form one continuous sound. Hence, the reflecting surface must be a 
certain distance from where the original sound is produced. The farther it 
h off", the longer the reflected sounds will be in reaching the observer's ear, 
and the more syllables will be repeated. At Woodstock, England, there is 
an echo which repeats from 17 to 20 syllables ; in this case the reflecting sur- 
face is distant about 2,300 feet. In mountainous regions echoes are quite 
common. There are several remarkable ones among the Alps ; and the 
mountaineers contrive to sing one of their national songs in such time that 
the echo forms an agreeable accompaniment. 

In ordinary rooms no echo is perceived, because the distance of the walls 
is so small that the reflected sound is mingled with the original one ; but in 
large halls, unless the principles of Acoustics are regarded, an unpleasant 
echo follows the speaker's words and makes them confused and indistinct. 

734. Ear-trumpets. — Ear-trumpets, used by deaf per- 
sons, concentrate and reflect to the interior membrane of 
the ear, vibrations that strike it, and thus render audible 
sounds that could not otherwise be heard. The principle 
on which they operate will be understood from Fig. 263. 

Fi°- 263 ^e soun ds enter the large end, and are united by 

successive reflections at the small end, which is applied 
to the ear. The outer part of the ear is itself of such a 
shape as to collect the sound-waves that strike it and re- 
flect them to the membrane within. To enable them to 
hear more distinctly, we often see people putting up their 
hands behind their ears, so as to form a concave reflect- 

THE EAR-TRUMPET. . 

mg surface ; in which case, the hand acts somewhat on 
the principle of the ear-trumpet. Instinct teaches animals to prick up their 
ears when they want to catch a sound more clearly. 

Shells of a certain shape reflect from their inner surface the vibrations 
that strike it from the external air, and hence the peculiar sound that is 
heard when they are applied to the ear. 

Echo "? In what case may a sound be repeated more than once ? How often does an 
echo near Milan repeat a syllable ? What is essential to the distinctness of an echo ? 
On what does the number of syllables repeated depend ? Give an account of the 
echo at Woodstock, England. Where are echoes quite common ? What is said of 
those in the Alps ? Why is there no echo in ordinary rooms ? 734. How is it that 
Ear-trumpets render audible sounds that could not otherwise be heard ? What is 
said of the outer part of the ear ? How is the hand made to act on the principle of 
a speaking-trumpet ? Why do animals prick up their ears? Explain the roaring of 




WHISPERING GALLERIES. 281 

735. Whispering Galleries. — Sound reflected from curved 
surfaces follows the same law as light and heat. Let two 
large concave brass mirrors be placed opposite to each 
other, as shown in Fig. 213 ; the ticking of a watch, or the 
fointest whisper in the focus of one, is distinctly heard, 
after two reflections, at the focus of the other, though in- 
audible at any other point. Two persons with their backs 
to each other can thus carry on a conversation, while those 
between them are not aware that anything is being said. 

An apartment in which such a reflection is produced by 
the walls is called a Whispering Gallery. An oval form is 
the best for such a gallery, because there are two points 
within, to either of which all the vibrations produced at 
the other are reflected at the same instant from every point 
of the surrounding walls. The dome of St. Paul's Church, 
London, and that of the Capitol at Washington, are exam- 
ples of fine whispering galleries. 

One of the most remarkable structures of this kind in ancient times was 
" the ear of Dionysius", a dungeon so called from the tyrant of Syracuse, by 
whom it was constructed. The walls and roof were so arranged that every 
sound from within was conveyed to a neighboring apartment, where the 
tyrant could hear even the whispers of his unsuspecting victims. 

736. The Phonograph is an instrument recently in- 
vented, for recording sounds and reproducing them at any 
time afterward. For description, see Appendix, p. 451. 

737. Musical Sounds. — Musical Sounds are produced 
by regular vibrations, uniform in duration and intensity. 
In connection with them we must consider three things — 
Loudness, Pitch, and Quality. 

The Loudness of a musical sound depends on the am- 
plitude of the vibrations producing it. The greater the 
vibrations, the louder is the sound. 

The Pitch of a musical sound depends on the rapidity 

shells. 735. What law does sound reflected from curved surfaces follow ? Illustrate 
this law in the case of sounds reflected from two concave mirrors. What is a Whis- 
pering Gallery? What is the hest form for such a gallery, and why? What build- 
ings contain whispering galleries ? Give an account of "the ear of Dionysius". 736. 
What is the Phonograph ? 737. How are Musical Sounds produced ? In connection 
with them, what must be considered? On what does the Loudness of a musical sound 



282 ACOUSTICS. 

of the vibrations producing it. The more rapid the vibra- 
tions, the higher is the pitch. 

The slowest vibrations that produce audible musical sounds follow each 
other at the rate of 8 in a second, and a very low note is the result. As the 
vibrations become more rapid the pitch rises, till they recur at the rate of 
24,000 in a second, when a very high note is produced. Beyond this the vi- 
brations last so short a time that they no longer affect an ordinary ear, and 
no musical sound is heard. 

The Quality of a musical sound depends on the nature 
of the vibrating body. The human voice, the piano, and 
the flute, may all produce a note of precisely the same 
loudness and pitch, and yet we readily distinguish them 
apart. The difference lies in their Quality. 

738. All musical sounds are produced by the regular 
vibrations either of solids or confined air. This gives rise 
to a division of musical instruments into two classes : — 
Stringed Instruments, like the violin ; and Wind Instru- 
ments, like the flute. 

739. Stringed Instruments. — The strings used in mu- 
sical instruments are made of metal or cat-gut. They are 
fastened at each end, and are set in vibration with the fin- 
ger, as in the case of the harp, — or by the stroke of a ham- 
mer, as in the piano, — or by drawing across them an instru- 
ment made for the purpose, like the bow of a violin. 

740. To produce notes of different pitch, two strings 
must vibrate with different degrees of rapidity. That they 
may do so, one must be longer than the other, or thicker, 
or stretched more tightly. 

The longer a string is, with a given thickness and tension, the more 
slowly it vibrates and the graver its tone. — The thicker a string is, with 
a given length and tension, the more slowly it vibrates and the graver its 
tone. — The more tightly a string is stretched, with a given length and thick- 
ness, the more rapidly it vibrates and the more acute its tone. 

depend ? On what, its Pitch ? How rapidly do the vibrations that produce the low- 
est audible musical sounds follow each other ? How rapidly, those that produce the 
highest notes ? On what does the Quality of a musical sound depend ? Give an ex- 
ample of difference in quality. 73S. By what are all musical sounds produced ? How 
are musical instruments, then, divided ? 739. Of what are the strings used in mu- 
sical instruments made ? How are they set in vibration ? 740. How are two strings 
made to produce notes of different pitch ? State the three laws relating to tne length, 



WIND INSTRUMENTS. 283 

Stringed instruments are tuned, — that is, brought to their proper pitch, — 
6y turning pegs to which the strings are attached. Changes in the condition 
of the atmosphere affect the length and consequently the tone of the strings. 

741. The music of the ^Eolian Harp is produced by the action of currents 
of air on strings which are stretched between two small uprights two or three 
feet apart. The most pleasing combinations of sounds sometimes proceed 
from this simple instrument, commencing with a strain, soft and low, as ii 
wafted to the ear from a distance, then swelling as if it were coming nearer^ 
while other notes break forth, mingling with the first with indescribable 
sweetness. 

742. In the case of the drum, musical sounds are produced by the vibrar 
tions of a tense membrane acting on the same principle as strings. 

743. Wind Instruments. — In wind instruments, such as 
the flute, the trumpet, &c, musical sounds are produced 
by the vibrations of air confined within tubes. In tubes 
of equal diameter, the pitch of the note differs according 
to the length of the vibrating column ; the shorter the col- 
umn, the higher or sharper the note. 

There are two ways of producing notes of different pitch 
with the same instrument : — 1. By joining tubes of dif- 
ferent length and diameter, as in the organ. 2. By having 
but one tube and providing apertures in it at different in- 
tervals, by uncovering which the air is allowed to escape, 
and the internal vibrations are stopped at any desired point. 
This is the arrangement in the flute. 

A wind and a stringed instrument produce notes of the same pitch when 
the column of air contained within the former vibrates with the same rapid- 
ity as the string which produces the note of the latter. 

744. The tubes of wind instruments may be open at both ends, or closed 
at both ends, or open at one end and closed at the other. In the last case, 
the note produced is twice as low as in either of the other cases, the length 
of the tubes being the same. 

745. Musical notes are produced with wind instruments by blowing into 
one end, by causing a current of air to enter an aperture, or by making 

thickness, and tension of strings. How are stringed instruments tuned ? What 
causes them to get out of tune ? 741. How are the sounds of the iEolian Harp pro- 
duced ? Describe the music of this instrument. 742. How are musical sounds pro- 
duced in the case of the drum ? 743. How are musical sounds produced in wind 
instruments ? On what does the pitch of the note depend ? How many ways are 
there of producing notes of different pitch with the same wind instrument? Mention 
them. When do a wind and a stringed instrument produce notes of the same pitch ? 
744 What is said respecting the openings of the tubes of wind instruments ? 
746. What three modes of producing musical notes with wind instruments are men- 



284 ACOUSTICS. 

such a current act on thin plates of metal or wood properly arranged 
within. 

746. A jet of hydrogen gas, ignited and made to pass through a glass tube 
about an inch in diameter, produces sweet musical sounds, which may be 
made soft or loud at pleasure by raising or lowering the tube. These sounds 
are caused by vibrations excited in the confined air by the burning hydrogen. 

747. The Organ. — The grandest and most complicated 
of wind instruments is the organ. It combines the tones 
of almost every other wind instrument, in such a way that 
they may be used singly or together at the pleasure of the 
performer. Organs, moreover, are now made with tones 
so closely resembling those of the human voice, that one 
who hears them would suppose he was listening to a full 
choir of singers. The great organ at Haarlem, in Holland, 
one of the most noted in the world, has no less than 5,000 
pipes, as the tubes of the organ are technically called. 

The water-organ, or hydraulicon, was known more than two hundred 
years before the Christian era. Its invention is attributed to Ctesibius, the 
barber of Alexandria, already mentioned as the inventor of the lifting-pump. 
Wind-organs appear to have been little known until the eighth century after 
Christ, though perhaps invented some time before. We read that an instru- 
ment of this kind was sent to King Pepin, of France, in the year 757, by the 
Greek Emperor, Constantine. 

748. The Gamut. — Notes are said to be in unison when 
the vibrations that produce them are performed in equal 
times. 

Two notes, one of which is produced by twice as many 
vibrations as the other, are called Octaves. In passing 
from a note to its octave, there are several intermediate 
sounds, produced by intermediate numbers of vibrations, 
each of which the ear recognizes as a distinct note. These 
notes are distinguished by different names, as shown be- 
low. Assuming the number of vibrations producing the 
first to be 1, the relative number of vibrations producing 



tioned? 746. How may musical notes be produced with a jet of hydrogen gas? 
747. What is the grandest of wind instruments? What are combined in the organ ? 
What tones are added in some instruments ? How many pipes has the great Haarlem 
organ ? How long ngo was the water-organ known ? By whom was it invented ? 
When do wind-organs appear to have first become known ? 748. When are notes 
said to be in unison? What is meant by Octaves ? Between a note and its octave, 



THE GAMUT. 285 

the other notes will be expressed by the fractions respec- 
tively placed below them, the number of the eighth note 
being, as already stated, double that of its octave. 

Names of the notes, CDEFGABC 

or, do re mi fa sol la si do 

Pronounced, do ra 'me fah sole lah %e do 

No. of vibrations, 1 f £ f § § - 1 /- 2 
These eight notes constitute the Gamut, or Diatonic Scale. The notes 
of the next higher octave bear the same relations to each other, but are pro- 
duced by vibrations performed in half the time, and therefore twice as nu- 
merous in each ca3e. The notes of the next lower octave again bear the same 
relations to each other, but their vibrations take twice the time, and are there- 
fore only half as numerous. In other words, a given note of any octave is 
produced by vibrations twice as rapid as the same note of the next octave 
below, and only half as rapid as the same note of the next octave above. 

749. Harmony. — Some notes, reaching the ear simul- 
taneously, produce an agreeable impression in consequence 
of their vibrations' frequently coinciding, and constitute 
what is called concord. Other notes, whose vibrations 
rarely coincide, impress the ear unpleasantly and produce 
discord. A combination of concordant musical sounds is 
called a Chord. An agreeable succession of musical sounds 
constitutes Melody. A succession of chords constitutes 
Harmony. 

The most agreeable concord is that of the octave ; next, 
the fifth ; then, the fourth ; and then, the third. Thus, in 
the scale given above, concord is produced when C is sound- 
ed with its octave C, and with the notes G, F, and E. 

750. The Human Voice. — The sounds of the human 
voice, whether used in speaking or singing, are produced 
by the vibrations of two membranes stretched across a 
tube, which connects the mouth with the lungs. This tube 
is the wind-pipe ; and the upper part of it, which consists 

what occur ? Name the notes by letters. Give their other names. Assuming the 
number of vibrations that produce C to be 1, mention the relative numbers that pro- 
duce the other notes. What do these eight notes constitute? What relation do the 
notes of the next higher octave bear to these ? The notes of the next lower octave ? 
749. What is meant by Concord ? By Discord ? What is a Chord ? What is Melo- 
dy? What is Harmony? Which is the most agreeable concord? Which next? 
Which next ? 750. How are the sounds of the human voice produced ? Describe 



286 



ACOUSTICS. 



Fig. 264. 



of cartilage, is called the Larynx. The larynx is flattened 
at the top, and terminates in two membranes, which nearly 
close the passage, leaving between them a narrow opening, 
known as the Glottis. These two membranes are called 
the Vocal Chords, and it is by their vibration, caused by 
the passage of the air breathed out from the lungs, that the 
sounds of the voice are produced. Small muscles enable 
lis to stretch the vocal chords more or less tightly at pleas- 
ure, and also to enlarge or diminish the opening between 
them. By these means we produce notes of different pitch. 
To produce a change of note, we have only to make a dif- 
ference of tjV o °f an mcn m the length of the vocal chords. 

Fig. 264 represents the glottis under differ- 
ent circumstances. The upper plate shows it 
at rest : b, b, represents the top of the larynx, 
and c, c, the vocal chords, relaxed so that the 
breath passing through the opening makes no 
sound. The lower plate shows the glottis in the 
act of emitting a musical sound, the chords be- 
ing now tightly stretched, and made to vibrate 
by the air breathed out between them. is a 
passage leading into the wind-pipe, which re- 
mains open, however close to each other the 
chords may be brought. 

751. The vocal chords are shorter in boys 
and women than in men ; hence the voices of 
the former are sharper or higher than those of 
the latter. When boys reach the age of 14 or 15, 
the vocal chords rapidly enlarge, and the voice 
is said to change. — The more forcibly the air is 
expelled from the lungs through the wind-pipe and larynx, the louder is the 




THE GLOTTIS AND VOCAL 
CHOKDS. 



752. His surprising flexibility of voice enables man to imitate almost ex- 
actly, not only the cries of birds and beasts, but also the sounds of various 
musical instruments. This was shown by the performances of a band of 
twelve Germans a short time since in the principal cities. Each imitated a 
different instrument with his voice, and so accurately, that those who heard 



the Larynx and the Glottis. What are the membranes stretched across the top of 
the larynx called ? How do we produce notes of different pitch ? How great a dif- 
ference in the length of the vocal chords produces a change of note ? Point out the 
different parts in Fig. 264. 751. Why are the voices of men deeper than those of 
boys and women ? What causes the voices of boys to change t On what does the 
loudness of the voice depend? 752. What is said of the flexibility of the human 



THE HUMAN VOICE. 287 

them could hardly believe they were not listening to an instrumental 
concert. 

753. Ventriloquism. — Some persons by practice become 
able to utter sounds and words without moving the muscles 
of the face. When, besides this, by imitating the effect 
of distance, they can make the sounds they produce seem 
to come from some other object, they are called Ventrilo- 
quists. The illusion is sometimes complete. 

Amusing exhibitions of ventriloquism are often given, in which the per- 
former imitates to perfection the buzzing of bees, the grunting of pigs, the 
spitting of cats, the chirping of crickets, the drawing of corks, the gurgling 
of liquids, the moaning of the wind, the puffing of a locomotive, the cry of a 
young infant, conversation between different parties represented as approach- 
ing or receding, in different parts of the room, under tables, &c. — It is sup- 
posed that the priests of the ancient oracles practised ventriloquism, and 
tbus made their responses appear to come from shrines, statues, &c. 

754. Stammering. — Stammering is a defect in speech 
caused by the organs' not performing their respective parts 
in regular succession. A convulsive nervous action inter- 
feres with their operation. 

755. The difficulty in the case of deaf mutes does not 
lie in any imperfection of the organs, but proceeds simply 
from their deafness. Having never heard their own voices 
or those of others, they are utterly unable to appreciate 
sounds or adjust the organs properly for their articulation. 

756. Voices of the Inferior Animals. — Man alone has 
the power of articulation. The inferior animals utter cries 
of different kinds, according to the conformation of the lar- 
ynx and the nasal cavities connected with it. Some of the 
cat's tones very closely resemble those of the human voice. 

The sounds of insects are produced in various ways, — by the rapid vibra- 
tion of their wings, the rubbing of their minute horns against each other, 
the striking of their organs on the bodies around them, &c. 

757. The Human Ear. — The human ear consists of 
three distinct parts ; the outer ear, the drum, and • the in- 
voice ? What instance of its remarkable flexibility is given ? 753. What is Ventril- 
oquism ? Describe some of the feats of ventriloquists. What use is supposed to 
have been made of ventriloquism in ancient times ? 754. What is the cause of Stam- 
mering? 755. Why are deaf mutes unable to use their voices ? 756. What is said 
of the tones of the inferior animals? How are the sounds of insects produced? 




288 ACOUSTICS. 

ner ear. These parts and their connections are represented 
in Fig. 265. 

p. 2 g5 A A is the outer ear, which acts on 

the principle of the ear-trumpet, collect- 
ing the sound-waves and reflecting them 
along the pipe B to the membrane C, 
called the membrane of the tympanum. 
E is the tympanum or drum, bounded by 
the membrane C on the one side, and the 
membrane F on the other, and filled with 
air, which it receives from the tube D, 
the human eab. communicating with the mouth. G, the 

inner ear, contains a number of ducts, and is filled with a liquid in which 
the acoustic nerve floats. 

The sound-waves transmitted from the outer air cause the membrane C 
to vibrate C excites vibrations in the air confined in the drum, and this in 
turn causes F to vibrate. The liquid in the inner ear receives the vibrations 
from the membrane F, and transmits them to the acoustic nerve, by which 
they are conveyed to the brain, and the sensation of hearing is produced. 
When a person takes cold, the tube which connects the drum with the mouth 
is apt to be obstructed, and temporary deafness is the consequence. 

EXAMPLES FOR PRACTICE. 

1. (See § 724.) If the air were perfectly still and uniform in density, how would 

the report of a musket heard by a person 50 feet off compare in loudness 
with the same report heard at a distance of 250 feet ? 

2. A cannon is heard a quarter of a mile off with a certain degree of loudness. 

How far must a person be removed, to hear it with only */ lQ0 of its former 
distinctness ? 

3. (See § 725.) How far does sound travel through air, at a temperature of 

60° F., in 10 seconds ? In 20 seconds ? In one minute ? 

4. How much faster does the sound produced by the discharge of a cannon 

travel, than that produced by the snapping of a whip ? 

5. (See § 726.) I see the flash of a cannon two seconds before I hear its re- 

port. How far is it off? 

6. A clap of thunder does not reach the ear till four seconds after the accom- 

panying flash is visible. How far off is the thunder-cloud? 
/*. A thunder-cloud is distant about one mile. How many seconds will elapse 

between the flash and the clap ? 
8. (See § 727.) About how many feet will sound travel through water in 10 

seconds ? Through iron ? Through wood ? 

757. Name the parts of which the human ear consists. "With the aid of Fig. 265, 
point out the different parts, and show the operation of the organ. Why is tempo- 
rary deafness produced by a cold ? 



ELECTRICITY. 289 



CHAPTER XYI. 

ELECTRICITY. 

758. If a dry glass tube or a stick of sealing-wax be 
rubbed with a piece of flannel, and then held a short dis- 
tance above some shreds of cotton, they will be instantly 
attracted to it, and after adhering to its surface for an in- 
stant again thrown off. A peculiar odor is perceived ; and 
the face, when brought near the glass or wax, feels as if a 
cobweb were in contact with it. If the tube or sealing-wax 
be presented to a metallic body in a darkroom, a spark, 
accompanied by a sharp cracking sound, will be seen dart- 
ing from it to the metal. 

The force thus developed by friction is called Elec- 
tricity. The body in which it is developed is called an 
Electric, and is said to be excited or electrified. The at- 
traction exerted by the excited electric over light bodies 
is called Electrical Attraction. The substance by whose 
friction the electric is excited is known as the Rubber. 

759. Electricity as known to the Ancients. — The 
term electricity is derived from the Greek word electron, 
amber, the property in question having been first observed 
in that substance. 

Thales, one of the seven wise men of Greece, who flourished 600 years 
b. c, is said to have discovered electricity in amber ■ Theophrastus and 
Pliny, at a later date, speak of the attraction of amber for leaves and straws. 
Both Pliny and Aristotle were acquainted with the electrical properties of tbc 
torpedo ; and we are informed that a freedman of the Emperor Tiberius cured 
himself of gout by the use of its shocks. Yet the ancients appear to have 
known nothing more than a few isolated facts connected with the subject ; 
and as a science Electricity had no existence till the commencement of the 
seventeenth century. 

758. If a glass tube or a stick of sealing-wax be rubbed with flannel, what phe 
nomena will be observed ? Name and define the terms used in connection with this 
experiment. 759. What is the derivation of the term electricity ? What allusions are 
made to this property by ancient authors ? When did electricity originate as a soi* 



290 ELECTRICITY. 

760. Nature of Electricity. — Electricity is now re- 
garded as a mode of force operating on ordinary matter, 
the molecules of which it polarizes, or arranges in a definite 
direction. It is convertible into the other modes of force, 
— heat, light, magnetism, and chemical action. 

Electricity, like heat, was formerly supposed to be an 
exceedingly subtile fluid residing between the atoms of 
bodies ; and, as a matter of convenience, the expressions 
electric fluid, electric current, and others implying a mate- 
rial existence, are still retained. In using these terms, 
however, we must remember that electricity is not a form 
of matter, but of force. 

Before the acceptance of the Dynamic Theory, as taught 
by Tyndall, Grove, and others, the two leading theories 
respecting the nature of electricity were Du Fay's and 
Franklin's. 

Du Fay's Theory.— -Du Fay, a French philosopher, held that there are 
two distinct electric fluids (named by him Vitreous and Kesinous), each of 
which attracts the other, but exbibits repulsion among its own particles. 
That in their natural state these fluids pervade all bodies in equal quan- 
tities, and combining nullify each other ; that it is only when this quies- 
cent compound fluid is decomposed by friction, or some other agency, that 
electrical phenomena are exhibited. 

Franklin's Theory. — Dr. Franklin, whose views were once generally 
received by scientific men, believed that there is but one electric fluid, of 
which every body in its natural state possesses a certain quantity. That 
no evidences of the existence of this fluid are observed as long as a body 
retains its natural quantity ; but, when it has either more or less than this, 
it exhibits certain phenomena and is said to be electrified. When over- 
charged, a body exhibits the phenomena displayed by glass when excited 
by flannel, and to such an electrical condition Franklin applied the term 
Positive ; when deprived of its proper share, its phenomena are the same 
as those of excited resinous substances, and such an electrical state he 
called Negative. When a positive and a negative body are brought into 
communication, the former shares its superfluous electricity with the latter, 
till equilibrium is established. Du Fay made the difference between the 
two electricities to consist in quality ; Franklin, in quantity. 

761. Sources of Electricity. — Electricity is devel- 

ence ? 760. What is electricity now thought to be ? Into what is it convertible ? 
What was it formerly supposed to be ? What two theories respecting •ihe nature of 
electricity formerly prevailed ? Givo the substance of Du Fay's theory. Of Frank- 



ELECTRICAL ATTRACTION AND REPULSION. 



291 



oped — 1. By friction. 2. By chemical action. 3. By mag- 
netism. 4. By heat. 



rx 



Electricity developed. l>y Friction. 

762. Friction is one of the commonest sources of elec- 
trical excitement. Every one has noticed how his hair 
crackles under the comb in frosty weather. The same 
sound is heard on stroking the back of a cat, and if the 
room is dark sparks may be drawn from its fur. 

A striking example of the exciting power of friction is often afforded in 
factories. The endless hands by their friction on the wheels develop elec- 
tricity in great abundance, sometimes yielding sparks at a distance of two 
or three feet. In the carding-rooms of cotton-mills, fibres of cotton are 
kept dancing to and fro by alternate attractions and repulsions, so that 
steam has to be let in from time to time to dissipate the electric fluid. 

763. Electrical Attraction and Repulsion. — We 
have already noticed the alternate attraction and repulsion 
of shreds of paper, cotton, and similar sub- -Pig- 266. 
stances by excited electrics. These phenom- 
ena may be further exhibited with the appa- 
ratus represented in Fig. 266, which consists 
of a pith-ball suspended from a pillar by a 
long silken thread. 

Experiment 1. — Eub a glass tube with flannel, and pre- 
sent it to the pith-ball ; the latter will be instantly attract- 
ed to the tube. After they have remained in contact an 
instant, the ball will be thrown off. If we now present 
the tube a second time, the ball, instead of being at- 
tracted, will be repelled. After touching the ball with 
the finger, to deprive it of the electricity it has received 
from the tube, repeat the experiment with an excited stick 
of sealing-wax, and the same phenomena will be exhib- 
ited,— that is, the ball will at first be attracted, 
but on the second application of the wax will be 
repelled. We find, then,— 1. That both the glass 
and the sealing-wax attract the ball before they have imparted to it any 
of their own electricity. 2. That, after so doing, they both repel the ball. 

lin's. 761. By what is electricity developed? T62. What familiar instances are men- 
tioned of the production of electricity by friction ? What striking example is afforded 
in factories ? 763. What does Fig. 266 represent ? What may it be nsed to illustrate ? 
Describe the first experiment with the pith-ball. What two facts are shown by thia 



6 



Z^ 




292 ELECTRICITY. 

Experiment 2.— Suspend two pith-balls from a pillar by silk threads, and 
pj-eeent to them an electrified glass tube or piece of sealing-wax. They 
.Fig. ztil. will both be attracted ; but, on withdrawing the elec- 

tric, instead of hanging vertically, they will repel 
each other, as shown in Fig. 267. 

Experiment 3. — Excite the glass tube, present it 
to the ball represented in Fig. 266, withdraw it after 
a second or two, and then present the excited sealing- 
wax. The ball, instead of being repelled, is now at- 
tracted. Eeverse the experiment by presenting first 
the excited wax and then the glass, and the latter in 
like manner will be found to attract the ball. 

764. From these experiments we con- 
clude that there are two kinds of electri- 
cal excitement, which may be distin- 
guished as Positive and Negative. We may lay down 
the general law that substances charged with opposite elec- 
tricities attract each other ^ while those charged with like 
electricities repel each other. 

765. Why the electricity of one body when excited is 
positive, and that of another negative, we can not tell. 
There is no law by which it can be determined, before ex- 
periment, what kind of electricity a body will exhibit. In- 
deed, the same body exhibits different kinds when rubbed 
by different substances. Thus, polished glass is positively 
electrified, when excited with flannel, but negatively when 
rubbed on the back of a cat. Rough glass is negatively 
electrified whtn rubbed with flannel, but positively when 
excited by dry oiled silk. 

766. Electricity is confined to the surface of an excited 
body ; it does not extend to the interior. A hollow ball 
may therefore contain just as much electricity as a solid 
ball of the same size. 

767. Positive electricity is never produced without 
negative, or negative without positive. 

experiment? Describe the second experiment. The third experiment. 764. What has 
been inferred from these experiments ? What general law may be laid down ? 765. 
Why is the electricity of one body positive, and that of another negative ? What is 
said of the electricity of a body when rubbed by different substances? 766. In 
what part of a body does its electricity reside ? 767. By what is the production of 






CONDUCTION OF ELECTRICITY. 293 

When a glass tube is excited, the rubber is negatively electrified ; and 
positively, when sealing-wax is excited. This may be shown by applying 
the rubber to a pith ball charged with the electricity which it has excited 
either in glass or sealing-wax. The ball is invariably attracted, which shows 
that the electricity of the rubber is opposite to that of the electric it has ex- 
cited. 

768. Electrics and Non-electrics. — All bodies can be 
electrified, but not with equal facility. Those that are easily 
excited, are called Electrics ; those that it is hard to excite, 
Non-electrics. The metals generally are non-electrics. 

769. Conduction of Electricity. — If we touch the two 
pith balls represented in Fig. 267 as repelling each other 
(because charged with the same electricity) with a glass 
rod, they will continue to repel each other ; but, if we touch 
them with a metallic rod, they will fall and hang vertically. 
This is because glass does not draw off their electricity, 
while metal does. Some substances, therefore, conduct 
electricity, while others do not. 

Substances that transmit electricity freely are called 
Conductors ; those that do not, Non-conductors. 

As a general thing, the non-electrics are conductors, and the electrics non- 
conductors. Some of the chief conductors are the metals (silver and copper 
ranking among the best), charcoal, water, snow, living animals, flame, smoke, 
and steam. Among the principal non-conductors are gutta percha, shellac, 
amber, the resins, sulphur, glass, transparent gems, silk, wool, hair, feath- 
ers, dry paper, leather, baked wood, air, and gases generally. 

Good conductors, when brought in contact with excited bodies, at once 
draw off" their electricity, and transmit it to all parts of their own surface, 
however extended. Bad conductors, on the other hand, receive electricity 
slowly, and diffuse it over their own surfaces no less slowly. A good con- 
ductor connected with the earth or a body of water, does not for an instant 
retain electricity communicated to it, but merely serves as a highway for its 
passage to either of those media. 

770. Insulators. — The best non-conductors are called 



one kind of electricity always accompanied ? How may this be shown ? 768. What 
»re Electrics? Non-electrics? To which of these classes do the metals belong? 
769. How may it be shown that there is a difference in the conducting power of dif- 
ferent substances? What is a Conductor of electricity? A Non-conductor? To 
which of these two classes do the electrics generally belong ? To which, the non- 
electrics ? Mention some of the chief conductors. Some of the principal non-con- 
ductors. Show the difference between good conductors and bad conductors, when 
brought in contact with excited bodies. What is said of good conductors connected 



294 



ELECTRICITY. 



Insulators, because they insulate electrified bodies, — that is, 
cut off their communication with such objects as would 
withdraw their electricity. The air is an insulator ; were 
it not, no substance could remain electrified for an instant. 
When insulated, an excited body retains the electricity 
communicated to it, and is said to be charged. The- pith 
ball in the experiment described in § 758 was insulated by 
the silk thread. Had it been suspended by a wire, the 
metal, being a good conductor, would have withdrawn 
the electricity from the ball as fast as it was received, and 
none of the phenomena that followed would have been 
exhibited. 

Even when insulated, excited bodies will in time part with their electric- 
ity. This is because no insulation can be perfect. — Air, when imbued with 
moisture, acquires conducting power ; and hence in damp weather it is im- 
possible to keep an electric excited for any length of time. Well insulated 
bodies, slightly excited, may be kept several months in a dry atmosphere 
without any perceptible loss of electricity. 

771. Path of an Electric Current. — An electric cur- 
rent always follows the best conductor, and of two equally 
good it takes the shorter. 

772. Velocity of Electricity. — Various experiments 
have been made to determine the velocity of electricity. 
Their results show that electricity travels from 11,000 to 
288,000 miles in a second, according to its intensity and 
the nature of the conductor along which it passes. In the 
case of the velocity last mentioned, which far exceeds that 
of light, and is so great as to be absolutely inconceivable, 
the conductor was copper wire. 

773. Electrical Machines. — The Electrical Machine 
is an apparatus for developing large quantities of electricity 
by the friction of a rubber on a glass surface. Two kinds 
of electrical machines are in use, known as the Cylinder 



with the earth or a body of water ? 770. What is meant by Insulators ? Why are 
they so called ? Give an example of an insulator. When is an excited body said to 
be charged t Give an example. How is it shown that no insulation is perfect ? 
Show the difference in conducting power between dry and damp air. 771. What path 
is always taken by an electric current ? 772. How great is the velocity of electricity ? 
*73. What is the Electrical Machine ? How many kinds of electrical machines aro 



ELECTRICAL MACHINES. 



295 



and the Plate Machine, — a glass cylinder being used in the 
former, and a circular plate of glass in the latter, 

774. Experiments in electricity were originally performed with a glass 
tube rubbed with fur or flannel. Otto G-uericke, the inventor of the air-pump, 
was the first to contrive a machine for developing the fluid more abundantly. 
It consisted of a globe of sulphur, turned with a winch, and submitted to the 
friction of the hand. Newton substituted a glass globe for the sulphuc, 
About the middle of the eighteenth century, two further improvements were 
made, — the use of a rubber instead of the hand, and. the addition of a metal- 
lic conductor. 

775. The Cylinder Machine. — In the cylinder machine, 
represented in Fig. 268, electricity is developed by the fric- 
tion of a rubber upon a glass cylinder, usually from 8 to 12 
inches in diameter, supported between two uprights of well- 
dried wood, and made to revolve by a couple of wheels, as 
shown in the Figure, or (as is now generally preferred) by 
a simple winch attached to one end of the cylinder. 

Fig. 268. 




THE CYLINDEB ELECTRICAL MACHINE. 



m use ? What constitutes the difference between them ? 774. With what were ex- 
periments in electricity originally performed ? Who first contrived an electrical ma- 
chine? Describe Guericke's apparatus. What improvement did Newton make? 
What improvements were made about the middle of the eighteenth century? 



296 ELECTRICITY. 

A is the cylinder. The rubber, B, is a leather cushion stuffed with horso 
hair, and set on a spring which makes it press equally against the cylinder 
in all parts of its revolution. The intensity of its pressure is regulated by 
a sliding base-board, H, which can be moved by a screw towards or from 
the cylinder. Connected with the back of the rubber is the negative con- 
ductor, F, a hollow metallic cylinder, with round ends, insulated by a glass 
pillar. On the opposite side is a similar metallic cylinder, C, insulated in 
the same way, and called the prime conductor. Attached to this ia a rod 
bearing a row of metallic points, E, like the teeth of a rake, projecting to- 
wards the cylinder and reaching to within a short distance of it. Several 
holes of different size are made in the upper surface of the prime conductor, 
to admit of the introduction of different pieces of apparatus used in experi- 
menting. To prevent the electricity from escaping in the air before it reaches 
the prime conductor, a flap of black silk, G (which is a non-conductor), ex- 
tends from the upper edge of the rubber, across the top of the cylinder, to 
within an inch of the metallic points. 

776. Operation. — When the machine is to be used, its parts must be per- 
fectly clean and dry. The rubber is rendered more efficient by spreading on 
it a thin coat of an amalgam of zinc, tin, and mercury, mixed with lard. The 
screw must be adjusted so that the rubber may press with moderate force on 
the glass, and the prime conductor so placed as to bring the metallic points 
about an eighth of an inch from the cylinder. If positive electricity is re- 
quired, the negative conductor must be connected with the earth by a me- 
tallic chain. This done, the handle is turned. Positive electricity is soon 
developed on the surface of the revolving glass, and in the rubber negative 
electricity, which is carried to the negative conductor. The positive elec- 
tricity of the cylinder, on reaching the metallic points, affects the prime 
conductor by induction (§ 806), and draws its negative electricity across 
the metallic points, while the positive electricity of the prime conductor is 
repelled to its opposite side. The negative electricity received from the 
prime conductor neutralizes the positive electricity of the cylinder ; but, 
as the rubber is constantly receiving fresh supplies from the earth through 
the conducting chain, the process is kept up, and positive electricity is 
accumulated in the prime conductor, — not that the latter receives any from 
the cylinder, but because its own negative electricity is withdrawn. 

If negative electricity is wanted, the chain connecting the machine with 
the earth must be attached to the prime conductor instead of the negative 
conductor, and the required electricity can then be drawn from the latter. 

Water being a good conductor, if the air is damp the electricity is dissi 
pated almost as soon as it is developed. This may be prevented by placing 
under the cylinder a small box containing a bar of red-hot iron. The radia- 
tion of heat from the bar keeps the atmosphere around the machine dry. 

775. How is electricity developed in the cylinder machine ? With the aid of Fig. 268, 
point out the different parts of the cylinder machine. How is the electricity prevent- 
ed from escaping hefore it reaches the prime conductor ? 776. Describe the operation 
of the cylinder machine. If negative electricity is wanted, what must be done ? 
What is the effect of dampness on the working of the machine ? How is this dim- 



ELECTRICAL MACHINES. 



297 



777. When the machine is working, present your knuckle 
to the prime conductor; a spark, accompanied by a sharp 
crackling sound, darts to your hand, producing a pricking 
sensation. This is called the Electric Spark. Any conductor 
will draw off a spark ; but let a non-conductor, such as a 
piece of glass, be presented, and no spark will be received. 

778. The Plate Machine. — In the Plate Machine, a cir- 
cular plate of glass is used instead of a cylinder. Plates 
six and seven feet in diameter have been employed, with 
such power that a spark from jfcheir immense conductors 
is nearly sum- Fig. 269. 

cient to fell a 
man to the earth. 
Plate machines 
were the most 
powerful known, 
until the inven- 
tion of Holtz's 
machine (shown 
on p. 439), which 
uses two plates, 
parallel and very 
near to each oth- 
er, and develops 
electricity exclu- 
sively by induc- 
tion (§ 806). 

Fig. 269 represents 
the plate machine. A 
A is the plate, sup- 
ported on an axis be- 
tween two uprights 
and turned by the 
handle D. The plate the plate electrical machine. 

is pressed by two pair of elastic rubbers, fastened inside of the uprights. 




culty removed ? 777. When a knuckle is presented to the prime conductor, what 
follows ? If a non-conductor is presented, what takes place ? 778. In the Plate Ma- 
chine, what is used ? How large plates are sometimes employed ? What is said 
of the power of plate machines ? With Fig. 269, describe the plate machine. 



298 



ELECTRICITY. 



Fig. 270. 



E E E is the conductor, which consists of three long brass tubes joined at 
right angles, with large balls at intervals. Opposite the centre of the plate, 
two brass arms, B, C, provided with rows of teeth, extend on each side from 
the upright conductor. The plate being made to revolve by means of the 
handle D, the same results follow as in the case of the cylinder machine. 

779. The Insulating Stool. — The Insulating Stool 
consists of a platform of well-baked wood, supported on 
glass legs covered with varnish. A person on the stool, 
brought in connection with the prime conductor of a ma= 
chine by holding in his hand a chain proceeding from it, 
may be charged with positive electricity. Sparks may be 
drawn from his person, and his hair, if fine and dry, will 
stand on end. If he holds in his hand a silver spoon full 
of alcohol, another person not on the stool may set the 

spirits on fire by simply pre- 
senting his finger to it, and 
thus producing a spark. The 
insulating stool is used when 
electricity is medically applied. 
780. The Discharger. — 
The Jointed Discharger, Fig. 
270, is an instrument with 
which an operator can dis- 
charge a conductor without 
having any of the electricity 
pass through his person. It 
consists of a couple of curved 
brass rods, terminating in balls 
at one end and at the other 
jointed and fixed in a socket, by which they are attached to 
a glass handle. The glass, being a non-conductor, cuts 
off communication with the operator's hand. 

The Universal Discharger, represented in Fig. 271, is 
an instrument for passing a charge of electricity through 
any substance. Two wires, mounted on insulating pillars, 
are connected respectively with the positive and the nega- 




THE JOINTED DISCHARGER. 



T79. Of what does the Insulating Stool consist ? How is it used ? 780. What is the 
Jointed Discharger ? Of what does it consist ? What is the Universal Discharger ? 



THE LEYDEN JAR. 



299 




THE UNIVERSAL DISCHARGER. 



Fig. 272. 



tive conductor of a machine. r\ Fi e- 271 

The substance to be operated 
on is placed on a stand be- 
tween two balls at the ex- 
tremities of these wires, and 
thus made a part of the elec- 
tric circuit traversed by the 
fluid when a discharge takes 
place. 

781. The Leyden Jar, or Vial. — The 
Leyden [U'-denj Jar is a glass vessel used for 
accumulating electricity. It is so called from 
having been first used at Leyden, Holland, in 
the year 1745. 

The ordinary Leyden jar (Fig. 272) consists of a glass 
vessel, coated inside and outside with tin-foil, to within 
about three inches of its mouth. It is closed with a dry 
varnished cork, through which passes a wire, terminating 
above in a brass knob, and below in a chain, which touches 
the inner coating. If the knob of such a jar be held within 
half an inch of the prime conductor when a machine is 
working, a succession of sparks will pass to the knob. In 
a short time they cease, and the jar is then said to be 
charged. The inside (being connected with the knob) is charged with posi- 
tive, and the outside with negative electricity, which are prevented from 
uniting by the non-conducting glass between them. 

If a person now grasp the outside of the jar with one hand, and touch the 
knob with the other, he will experience the peculiar sensation called " the 
electric shock", in his arms, and if the jar is large, through his chest. If, on 
the other hand, he apply one ball of the jointed discharger to the outer coat 
and the other to the knob, the jar will be discharged without his feeling any- 
thing, because his communication with the jar is cut off by the glass handle. 
A body through which a charge is to be sent must form part of the circuit 
between the inner and outer coating of the jar, so that a union of the positive 
And negative electricity can not take place without passing through it. — So 
much electricity is sometimes accumulated in a jar that a discharge takes 
place through the glass, making a hole in it and rendering the jar useless. 




LEYDEN JAR. 



Describe it and its mode of operation. 781. What is the Leyden Jar ? Why is it so 
called ? Of what does the ordinary Leyden jar consist ? How is the jar charged ? 
With what kind of electricity is the inside charged ? The outside ? How may the 
electric shock be taken ? How may the jar be discharged without the operator's tak- 
ing a shock ? What is essential in order that a charge may be sent through a body '? 



300 EL': 3TRICITY. 

Any number of persons may take a shock at once. Having joined hands 
in a circle, let the person at one end take hold of a chain connected with 
the outside of a jar, while the one at the other end touches the knob with 
a piece of wire. The painful sensation experienced when a shock is tak- 
en, is caused by the resistance which those parts of the body that are im- 
perfect conductors offer to the molecular changes that electricity produces. 
782. An interesting incident is related in connection with the experiments 
that led to the invention of the Ley den jar. Prof. Muschenbroeck, of Ley- 
den, observing that excited electrics soon lose their electricity in the air, de- 
termined to see whether he could not collect and insulate the fluid in a vessel 
of non-conducting glass, so that it might be kept locked up, as it were, ready 
for use. Accordingly, he introduced a wire from a prime conductor into a 
bottle filled with water. After the machine had been working some time, an 
attendant, holding the bottle in one hand, attempted to withdraw the wire 
with the other, when he of course received a shock, — so unexpected and so 
unlike any thing he had ever felt before, that it filled him with consternation. 
Muschenbroeck himself subsequently took a similar shock, which he de- 
scribed in a letter to a French philosopher. He says that he felt himself 
struck in his arms, shoulders, and breast, so that he lost his breath, and it 
was two days before he recovered from the effects of the blow and the fright. 
He would not, he adds, take a second shock for the whole kingdom of France. 
783. The Electrical Battery. — When a very heavy- 
charge is required, a number of jars, coated in the usual 
way, are placed in a box lined with tin-foil, which forms a 
Fig. 273. communication between their out- 

er coatings, while their knobs and 
consequently their inside coatings, 
are connected in the manner rep- 
resented in Fig. 273. From its 
powerful effects, such a combina- 
tion is called an Electrical Battery. 
By bringing one of the knobs in 

THE ELECTRICAL BATTERY. . . , . -. 

connection with a prime conductor 
all the jars may be charged as readily as one, care being 
taken to connect the outer coatings with the earth. The 
battery may be discharged in the same way as a single jar, 
but the operator must not let the charge pass through his 

What is the consequence if too much electricity is accumulated in ajar ? How may 
any number of persons take a shock at once ? By what is the painful sensation of at 
electric shock caused? 782. Eelate an incident connected with the invention of the 
Leyden jar. What did Muschenbroeck say of the electric shock ? 783. Describe the 
Electrical Battery, and its mode of operation. What effects may be produced by the 




ELECTRICAL EXPERIMENTS. 



301 



person. The shock of a powerful battery will kill a man 
and fell an ox ; even moderate discharges prove fatal to 
birds and the smaller animals. 

784. Experiments with the Electrical Machine. — 
With the electrical machine and different pieces of appara- 
tus that accompany it, a variety of experiments may be 
performed. 

785. Electrical Bells.— This apparatus (Pig. 274) il- Fig. 274 
lustrates electrical attraction and repulsion. Two bells 
are suspended from a frame, with a brass clapper be- 
tween them. One of these bells having been placed in 
connection with the prime conductor and the other with 
the ground, the machine is worked ; when the former 
becomes charged with positive and the latter with neg- 
ative electricity. The clapper is attracted to the posi- 
tive bell, strikes it, becomes itself charged by the con- 
tact, and is repelled till it strikes the negative bell. Its 
positive electricity is there drawn off, and it falls back, 
to be again attracted and repelled. The clapper is thus 
made to strike the bells alternately. 

786. The Electrical See-saw. — The Electrical See-saw 
(Fig. 275) operates on the same principle. A brass beam, with a light figure 
on each end, is suspended on an insulating pillar, in such a way as to allow 
its extremities to move freely up and 




ELECTRICAL BELLS. 



down. Two brass balls are sup- 
ported at opposite sides of the stand, 
not far from the ends of the beam, — 
the one on a glass pillar, the other 
on a metallic rod. The insulated 
ball is connected with the inner 
coating of a Leyden jar, and the 
other with its outer coating. No 
sooner is the jar charged than the 
figure near the insulated ball is suc- 
cessively attracted and repelled, and 
this causes the beam to teeter. In 
the same way motion may be com- 
municated to 
air, &c. 



Fig. 275. 




ELECTRICAL SEE-SAW. 

figure swinging, a floating swan, an insect suspended in tin 



787. Dancing Images. — On a metallic plate supported by some conducting 



bhock of a powerful battery ? 7S5. Give an account of the experiment with the Elec- 
trical Bells. 786. Describe the Electrical See-saw. To what may motion be com- 
municated on the same principle ? 787. Give an account of the experiment with the 



302 



ELECTRICITY. 



Fig. 276. 




Fig. 277. 



DANCING IMAGES. 



substance, place several light figures of pith o> 
paper, and three or four inches above them sus- 
pend another plate from the prime conductor. 
As soon as the machine is worked, the figures 
will rise and dance up and down from one plate 
to another in a ludicrous manner, as shown in 
Fig. 276. If the lower plate is insulated, when 
they return to it after having been drawn up, th 
surplus positive electrici- 
ty can not escape, and 
the dance ceases. 

788. Diverging Threads. 
— Figure 277 represents 
twenty fine linen threads, 
eight or ten inches long, 
tied together at each end. 
Attach them to a prime 
conductor, and on work- 
ing the machine, being all 
filled with electricity of 



the same kind, they will repel each other and assume 
an oval form. 

Fig. 278. 




DIVERGING THREADS. 



789. The Electrified Head. — On the same prin- 
ciple a head of hair is made to stand grotesque- 
ly on end, as shown in Fig. 278, by fixing the 
wire to which it is attached in one of the holes 
of a prime conductor. The hairs are charged 
with electricity of the same kind, and are there- 
fore in a state of mutual repulsion, fig. 279. 
Draw off the fluid by presenting a 
knife-blade, and they at once fall. 

790. TJie Electrical Pail. — Suspend 
from the prime conductor ,by a chain, 
a pail with a small hole in the bot- 
tom, and fill it with water. Before 
the machine is worked, the water falls 
from the hole drop by drop ; but, as 
soon as the water is charged with elec- 
tricity, it flows out in a stream, which 

in the dark seems to be of fire. This is owing to the repulsion 
excited in the particles of water by charging them with the same 
electricity. 

791. TJie Aurora Tube. — This apparatus shows the phenomena 




THE ELBCTRIFIED HEAD. 



Dancing Images. "Why do the images cease to move if the lower plate is insulated? 
788. What does Fig. 277 represent ? What takes place when these threads are at- 
tached to a prime conductor ? 789. Describe the experiment with the Head of Hair. 



ELECTRICAL EXPERIMENTS. 



303 




produced when electricity passes through a vacuum. It is Fig. 280. 

a glass tube, from two to three feet long, surmounted by a 
brass ball. This ball is supported on a wire, which passes 
into the tube through its air-tight top, and terminates a 
short distance below in a point. Inside of the tube, near 
the bottom, is another brass ball supported on a wire. The 
lower part of the tube is arranged so that it can be fitted 
to the plate of an air-pump, and is commanded by a stop- 
cock. Having thoroughly dried and warmed the tube, ex- 
haust it by means of an air-pump ; then, in a dark room, 
bring the upper ball in communication with a prime con- 
ductor. As soon as the machine is worked, the whole 
length of the tube is filled with a continuous stream of 
violet light ; which, on a small scale, strikingly resembles 
the Aurora Borealis, or Northern Lights. This is a lumi- 
nous appearance often visible in the north on clear and 
frosty nights, and peculiarly vivid in high latitudes. It is 
supposed that the Northern Lights are produced by the 
passage of currents of electricity through strata of highly 
rarefied air. 

792. luminous Words. — When the con- 
tinuity of a conductor is broken, a spark darts 
from one part of it to another. Taking ad- 
vantage of this fact, we may perform a vari- 
ety of experiments, which in a dark room have 
a striking effect. 

Fig. 281. 0n a P iece 

of glass paste 

some strips of 

tin-foil, with 

portions cut 

out so that 

the spaces may form letters, as 

shown in Fig. 2S1. Connect the 

first piece of foil with the prime 

conductor, and the last with the 

ground. When the machine is 

worked, sparks will pass between 

the different divisions of the foil, and the letters consequently appear like 



AUKOUA TUBE. 



How may the hairs be made to fall ? 790. Describe the experiment with the Electri- 
cal Pail. What causes the water to flow more rapidly when the machine is worked ? 

791. What is shown with the Aurora Tube ? Of what does it consist ? Describe the 
experiment with it By what is it supposed that the Northern Lights are produced ? 

792. What takes place when the continuity of a conductor is broken ? By taking ad- 




304 ELECTRICITY. 

characters of fire. — Serpentine and spiral lines of light, and other beautiful 
appearances may be produced, by arranging spangles on glass in the de» 
sired form about one-tenth of an inch apart, and subjecting them to the ac- 
tion of the machine. 

793. The Electrical Pistol. — The electric spark may be 
made to explode a mixture of hydrogen and common air. 
In this experiment the Electrical Pistol (Fig. 282) is em- 
ployed. 

Fi r- 2S2 - The barrel of the pistol is of brass. 

* in i mi ifffrirjiiM i=TTy^ |h Where the trigger is usually found, is a 

short ivory tube, which insulates a wire 
passing nearly across the barrel, and ter- 
ras electrical pistol. minating on the outside in a ball. Hold 
the mouth of the pistol over a stream of hydrogen gas, and when enough has 
entered, close it with a cork. On passing a spark through the barrel from 
the extremity of the wire to the opposite surface, a loud report will be pro- 
duced, and the cork will be discharged with considerable force. 

794. Mechanical Effects of the Passage of Elec- 
tricity. — A pointed conductor receives and parts with 
electricity much more readily than one with a spherical 
surface. Hence, in electrical machines, points connected 
with the prime conductor are brought near the excited 
glass, while the prime conductor itself is cylindrical. 

Fix a pointed rod on the prime conductor, and a silent 
discharge will take place from it as long as the machine 
is worked. In this case, the prime conductor can not 
accumulate enough electricity to give a spark. In a dark 
room, the discharge from the point is made visible in the 
form of a luminous brush. A disturbance is produced in 
the adjacent air, which may be felt if the hand is brought 
near the rod, and is sometimes strong enough to blow out 
a candle. No such phenomena occur near the surface of 
the conductor or a ball attached to it. The point parts 
with its electricity more readily, charges the air in contact 
with it, and repels it when charged, as in the case of the 
pith- ball, — thus causing a constant current from the point. 

vantage of this fact, what beautiful experiments may be performed ? 793. For wha' 
is the Electrical Pistol used? Describe this instrument, and the experiment per- 
formed with it. 794. Why, in electrical machines, are metallic points connected with 
the prime conductor brought near the excited glass ? Why is the prime conductor 
itielf cylindrical? With what experiments is the silent discharge from points illus- 






MECHANICAL EFFECTS OF ELECTRICITY. 



305 



Fig. 283. 




795. The Phosphorus Cup. — An 
interesting experiment, showing 
the passage of an electric current, 
may be performed with the appara- 
tus represented in Fig. 283, known 
as the Phosphorus Cup. Two brass 
cups insulated on glass pillars are 
placed at the same height, about 
two inches apart, with a lighted can- 
dle midway between them. The 
cups, being each provided with a 
piece of phosphorus, are connected 
one with the prime conductor, and 
the other with the negative conductor, of a powerful machine. When the ma- 
chine is worked, the flame sets in the direction of the negative cup r towards 
which it is carried by a current of air charged from the opposite cup, and 
then repelled. The piece of phosphorus in the negative cup is soon set on 
fire by the flame, while the piece in the positive cup remains unignited. 
By reversing the connections with the machine, the opposite results may 
be produced, the flame being always carried towards the cup connected 
with the negative conductor. 

796. When electricity passes off from a pointed con- 
ductor, the reaction may be made to turn a wheel, axid 



THE frHOSPHORUB CUP. 



thus set delicate machinery in 
motion. To exhibit the effects of 
this reaction, different pieces of 
apparatus have been constructed, 
among which is the Electrical 
Flyer. 

The Electrical Flyer— -The Electrical 
Flyer consists of a number of brass wires 
branching out from a common centre, hav- 
ing their ends bent at right angles in the 
same direction. Poise the flyer on a wire 
inserted in the prime conductor, and work 
the machine. A discharge takes place from 
each point, setting in motion a current of 
air, by the reaction of which the flyer is 
made to revolve in the opposite direction. 



Fig. 284. 





THE ELECTBICAE FLYER. 



trated? Explain how a lighted candle is blown out by an electric current. 795. What 
does Fig. 283 represent? Describe this apparatus, and the experiment performed 
with it. Towards which cup is the flame always carried? 796. How may delicate 
machinery be set in motion ? How is this reaction shown ? Describe the Electrical 

20 



306 



ELECTRICITY. 



When the room is darkened, the points become luminous, and a circle of 
fire seems to be formed as they revolve. 

On the same principle, horsemen (mounted on the ends of the flyer) may 
be made to move in a circle ; wheels may be turned, the sails of a windmill 
set in motion, and a light body made to roll up an inclined plane. 

797. The Thunder House. — The power of electricity, as 
a mechanical agent, may be further illustrated with an in- 
genious apparatus known as the Thunder House. 



Fig. 285. 



A 



THE THUNDER HOUSE. 



The Thunder House consists of a piece of baked 
mahogany, B B, shaped like the gable of a house, and 
attached to a stand. Down the centre runs a wire, C, 
terminating above in a ball, A. Several square pieces, 
D, F, about one-fourth of an inch thick, are cut out of 
the gable, and placed loosely in the holes from which 
they are cut. Across each square passes a wire in such 
a direction that by inserting the squares one way we 
have an uninterrupted line from C to E ; but putting 
them in crosswise, we break the continuity of the con- 
ductor at D and F. Connect the end of the wire, E, 
with the outside of a Leyden jar ; and, having inserted 
the square so that the conducting line may be un- 
broken, pass a charge through the wire by connecting 
the ball A with the inside of the jar. A report will be 
heard, but neither of the loose pieces will be displaced. 
Now let one of the pieces remain in the same position, and place the other 
crosswise ; then, on passing a powerful charge through the wire, the former 
will remain undisturbed, while the latter will be thrown out of the gable by 
the mechanical action of the electricity in passing the break. 

798. Among the mechanical effects of an electric dis- 
charge may be mentioned the perforation of thin non- 
conducting substances, such as a card or a piece of paper. 
Glass one-twelfth of an inch thick may be pierced by a dis- 
charge from a powerful battery. 

799. The Electric Spark. — The color of the electric 
spark varies according to the medium through which it 
passes. In ordinary air and oxygen, it is bluish white ; 
in rarefied air, violet , in nitrogen, a purplish blue ; in 
hydrogen, crimson ; in carbonic acid and chlorine, green. 

Flyer. What is the effect of darkening the room ? To what may motion be com- 
municated on the principle of the flyer ? 797. What apparatus further illustrates the 
mechanical power of electricity ? Describe the Thunder House, and the experiment 
performed with it. 798. What other mechanical effect of an electric discharge 
is mentioned ? 799. What does the color of the electric spark depend on ? What 



THE ELECTRIC SPARK, 



307 



The length and intensity of the spark depend on the 
electrical intensity of the body from which it proceeds. 
Sparks may be taken from the prime conductor of a pow- 
erful machine at a distance of more than two feet. In a 
given machine, the positive conductor yields much more 
powerful sparks than the negative. 

800. Ignition by the Electric Spark. — Inflammable sub- 
stances may be set on fire by the electric spark, as is shown 
by several experiments. 

Stand on the insulating stool, touch the prime conductor with one hand, 
and from the other transmit a spark to a burner from which a current of gas 
is issuing, — the gas will be ignited. In houses thoroughly dried by furnace 
heat, persons, by simply running over the carpet, have been sufficiently 
charged with electricity to light gas with a spark from the finger. — Present a 
candle just extinguished, with its wick still glowing, to a prime conductor, 
so that a spark may pass through the snuff to the candle, and it will be re- 
lighted. — A person on an insulating stool charged with electricity may set 
fire to a cup of ether by presenting to it an icicle, through which the spark 
is transmitted. — With a suitable apparatus, a fine wire may be melted by 
sending through it a charge from a powerful battery. 

801. The Electrical Fire House. 
— Rosin may be ignited with the 
apparatus known as the Electrical 
Fire House (Fig. 286). Brass wires, 
insulated by being enclosed in 
glass tubes,enter the opposite sides 
of the house, and terminate on the 
inside in two knobs, B, C, a short 
distance apart. These knobs are 
loosely covered with tow and 
sprinkled with powdered rosin. 
When a charge is passed from A 
to D, the rosin is ignited, and the 
flame seen through the windows 
gives the house the appearance of 
being on fire. 

802. Apparatus for firing Gtmpowder 



Fig. 286. 




THE ELECTRICAL FIRE HOU8E. 



-This apparatus consists of two 



Js its color in ordinary air aad oxygen ? In rarefied air? In nitrogen? In hydro- 
gen ? In carbonic acid and chlorine ? What do the length and intensity of the spark 
depend on? At what distance have sparks been taken from a powerful machine? 
How do the sparks from the positive conductor compare with those from the nega- 
tive? 800. What is the effect of the electric spark on inflammable substances? 
Prove, this with several experiments. What is the effect of sending a powerful 
charge through u fine wire ? 801. Describe the Electrical Fire House, and the ex- 



308 



ELECTRICITY. 




Fig. 287. insulating glass pillars fixed in a stand, 

to one of which is attached a wire termi- 
nating in a ball, to the other a wooden 
cup for holding the powder. The chains 
c, d, being connected respectively with 
the inner and outer surface of a Leyden 
jar, a spark is made to pass from b to A, 
which ignites the powder. 

803. The Electrophorus. 
— Small quantities of electricity 
maybe accumulated with a sim- 
ple apparatus known as the Electrophorus, which to a cer- 
tain extent answers as a substitute for the electrical 
machine. 

The electrophorus consists of a cake of a resinous mixture 8 or 10 inches 
in diameter, and a somewhat smaller plate of metal with a rounded edge and 
a glass handle, by which it may be raised without drawing off the electricity. 
Excite the resinous mixture with fur, and placing on it the metallic plate, 
touch the upper surface of the latter for an instant to let its negative elec- 
tricity escape. Then raise the metallic plate by the insulated handle, and 
on presenting a conductor a spark will be given. Place the metallic plate 
again upon the rosin, and on raising it another spark may be withdrawn. A 
Leyden jar may thus be slowly charged. Left on the rosin, the metallic 
plate will remain charged for a long time, and may be conveniently used as 
occasion requires in experimenting. 

804. Electroscopes. — Electroscopes are instruments 
for detecting the presence of electricity, and determining 
whether it is positive or negative. They appear in various 
forms, — the simplest being the pith ball suspended by a 
silk thread, represented in Fig. 266. The attraction of the 
pith ball in its natural state by any substance presented to 
it, indicates the presence of electricity in the latter. When 
the pith ball is charged with positive electricity, its attrac- 
tion by any substance indicates negative electricity in the 
latter, and its repulsion positive. When the pith ball is 



periment performed with it. 802. Of what does the apparatus for firing gunpowder 
consist ? 803. With what may small quantities of electricity he accumulated ? 
what does the Electrophorus consist ? How is it worked? 804. What are Electro 
scopes ? What is the simplest form of the electroscope ? How is the presence of elec- 
tricity in any substance indicated? When the pith ball is positively charged, what 
does its attraction by any substance indicate ? What, its repulsion ? When the pith 



THE ELECTROMETER. 



309 



Fig. 288. 




QUADRANT ELEC- 
TROMETER. 



charged with negative electricity, its attraction by any sub- 
stance indicates positive electricity in the latter, its repul- 
sion negative. 

805. Electrometers. — Electrometers are 
instruments for measuring approximately the 
quantity of electricity in a given conductor 
or other body. Electrometers, more or less 
sensitive, are made in different forms ; one of 
the simplest is the Quadrant Electrometer, 
shown in Fig. 288. 

A slender ivory rod, with a pith ball attached to its 
lower end, is suspended from a wooden pillar so as to 
swing freely like a pendulum. The pivot on which it 
turns is the centre of a semicircular scale attached to the 
pillar ; and the whole apparatus terminates in a brass pin 
which may be inserted in the top of a prime conductor, 
The greater the quantity of electricity in the latter, the 
farther from the pillar the pith ball will swing, — and this 
distance is indicated by the scale. 

806. Electrical Induction. — An electrical atmosphere 
surrounds every excited body. An insulated conductor 
situated within this atmosphere becomes excited, and when 
thus affected is said to be electri- 
fied by induction. The phenom- 
ena of electrical induction are con- 
stantly exhibited. 

807. Electrical induction is illustrated with 
the apparatus represented in Fig. 289. cadis 
a brass cylinder with rounded ends, insulated 
on a glass support and furnished at one ex- 
tremity with a pith ball electroscope,/. On 
bringing the end d within a few inches of a 
prime conductor, the pith balls, which be- 
fore hung close together, instantly separate, 
indicating the presence of electricity. Since 
the cylinder is not in contact with the prime 




INDUCTION APPARATUS. 



tall is negatively charged, what does its attraction indicate ? What, its repulsion ? 
605. What are Electrometers ? What is one of the simplest forms called ? Describe 
the Quadrant Electrometer, and its mode of operation. 806. By what is every ex- 
cited body surrounded ? When is a body said to be electrified ~by induction f 
S07. Describe the apparatus for illustrating electrical induction, and the experiments 



310 ELECTRICITY. 

conductor and receives no sparks from it, it is obviously electrified by in- 
duction. The polarized molecules of the surrounding air transmit their 
polarization to the particles of the cylinder ; and, these being conductors, 
the whole cylinder is polarized as if it were a single particle. The part of 
the cylinder towards d is affected with negative electricity, the part towards 
c with positive ; it is by the latter that the two balls are charged and thus 
made to separate. If the cylinder is removed from the neighborhood of 
the prime conductor, the pith-balls immediately fall together. 

If the cylinder cad, instead of being insulated, is connected with the 
earth, its positive electricity is driven off to the latter, while the negative 
portion is retained. If the cylinder is then removed, its communication with 
the earth being first cut off, it will remain excited with negative electricity. 

808. Electricity from Steam. — Electricity is devel- 
oped during the escape of steam from an orifice. This fact 
was discovered in 1840 by a workman attending a steam- 
engine ; who, happening to take hold of the safety-valve 
with one hand while the other was in a jet of steam escap- 
ing from a fissure, received an electric shock. The experi- 
ment was repeated, and it was found that a person with 
one hand in a jet of escaping steam could give a shock with 
the other to any one in contact with the boiler or the brick 
work supporting it. The electricity in question is produced 
by the friction of minute particles of water against the sides 
of the orifice. 

As soon as this fact came to the knowledge of scientific men, an appara- 
tus known as the Hydro-electric Machine was invented for the purpose of 
experiment. It consists of a steam boiler from three to six feet long, mount- 
ed on insulating pillars, with an arrangement for letting the steam escape in 
jets against a plate covered with metallic points, which acts like a prime con- 
ductor. This machine develops electricity in prodigious quantities, its power 
being equal to that of four large plate machines combined. It yields' sparks 
22 inches long, in such quick succession that they resemble a sheet of flame. 

809. Atmospheric Electricity. — Electricity is at all 
times present in the atmosphere in a greater or less degree, 
being most intense, as a general rule, three or four hours 
after sunrise and sunset. The quantity of atmospheric 

performed with it ? How may the cylinder be charged with negative electricity f 
808. Under what circumstances is electricity produced by steam ? State the circum- 
stances attending this discovery. What was found when the experiment was repeat- 
ed ? How is the electricity in question produced ? What instrument was invented 
for the sake of further experiment? Describe the Hydro-electric machine. To what 
in its power equal ? What is said of its sparks ? 809. Where is electricity alway* 



ATMOSPHERIC ELECTRICITY. 311 

electricity increases with the distance from the earth's 
surface. This is proved by sending up arrows connected 
by a conducting metallic wire with a delicate electrometer. 
The higher the arrows rise, the more the electrometer is 
affected. An experimenter in England, by connecting a 
number of pointed conductors with an insulated wire a 
mile long and raised a hundred feet above the earth's sur- 
face, has collected enough electricity to charge a battery 
of fifty jars every three seconds. 

810. Origin. — The electricity in the atmosphere is due 
— 1. To the friction of large masses of air of different 
densities on each other. 2. To the condensation of atmos- 
pheric vapors into a liquid form — a process which develops 
electricity in great abundance. 3. To the chemical changes 
involved in the growth of trees and plants. 4. To evapo- 
ration, particularly in the case of water filled with vege- 
table matter undergoing decomposition. 

As these processes are not always going on with the 
same activity, the quantity of electricity present in the 
atmosphere differs at different times and places. 

811. St. Elmo^s Fire. — When the atmosphere is very 
abundantly charged with electricity, its presence is indi- 
cated by various luminous phenomena. Hence the brilliant 
light called St. Elmo's Fire, which frequently appears at 
night on the toj>s of masts, the points of bayonets, and the 
tips of the ears of horses. It is simply the superabundant 
electricity of the atmosphere, attracted by a pointed con- 
ductor, into which it silently passes. Such phenomena are 
most common during thunder-storms, when as many as 
thirty have been seen in different parts of the same vessel. 
Sometimes they resemble sheets of flame, and extend three 
feet in length ; at others, they take the form of globes 
of fire, attaching themselves to yard-arms and mast-heads. 

present ? When is it most intense ? In what regions of the atmosphere ? How is 
this proved ? What has heen done in this connection in England ? 810. To what 
fonr processes is the electricity in the atmosphere chiefly due ? Why is the quan- 
tity of electricity in the atmosphere different at different times? 811. When are 
luminous phenomena ohserved in the atmosphere ? Describe the phenomenon known 



312 ELECTRICITY. 

812. Aurora JBorealis, — The Aurora Borealis was men- 
tioned in § 791, as a luminous appearance in the heavens 
attributed to atmospheric electricity. It is often accom- 
panied with the crackling sound of electricity, and some- 
times with the peculiar odor which attends an electric dis- 
charge. Though most frequent and vivid in the polar 
skies, it also occasionally occurs in temperate latitudes. 
One of the most gorgeous forms of the Aurora is that of a 
cloud parallel with the horizon, fringed on its upper edge 
with light, from which bright streamers of different colors 
constantly shoot up to the zenith with a tremulous motion. 

813. Lightning and Thunder. — The grandest of all 
the phenomena produced in the atmosphere by electricity 
is Lightning. Lightning is nothing more than the spark 
which accompanies the passage of electricity from one 
cloud to another, or between a cloud and the earth. It 
sometimes appears in the form of a fiery ball, which moves 
through the atmosphere toward the earth and is seen for 
several seconds. Thunder is the crackling sound which 
accompanies the discharge. Flashes of lightning are some- 
times several miles in extent ; and, as the crackling sound 
is produced at every point of their course, it does not reach 
our ear all at the same instant. Hence the rolling or rum- 
bling of thunder, which is in some cases prolonged by suc- 
cessive echoes from neighboring mountains or clouds. 

814. That lightning and thunder are produced by an 
electric discharge, though previously suspected, was first 
experimentally proved in 1752, by Benjamin Franklin, 
whom the world recognizes alike great as a philosopher 
and a patriot. 

Impressed with, the conviction that lightning and the electric spark were 
identical, Franklin determined to test its truth by trying to collect electricity 

as St. Elmo's Fire. A.t what time is it most common ? What different forms does 
it ussume ? 812. What other phenomenon is attributable to electricity ? Describe 
one of the most gorgeous forms of the aurora. 813. What is the grandest of 
all the electrical phenomena of the atmosphere? What is Lightning? What is 
Thunder ? How is the rolling of thunder accounted for ? 814. By whom and when 
was it proved that lightning and thunder are produced by an electric discharge ? 



FRANKLIN'S EXPERIMENT. 313 

from the clouds during a thunder-storm. With this view he made arrange- 
ments for extending a wire to a great height from a steeple then in course of 
erection in Philadelphia. The work advanced but slowly ; and while anx- 
iously watching its progress one day, he observed a boy's kite far up in the 
air, and higher than he could hope to get his wire even when the steeple 
should be finished. It struck him at once that with this simple toy he could 
make the desired experiment, letting the string perform the part of the con- 
ducting wire. Accordingly, he made a cross of two strips of cedar, to the 
extremities of which he fastened the four corners of a silk handkerchief, 
,jsing this as a covering that his kite might be able to withstand the rain and 
wind accompanying a thunder-shower. A sharp-pointed wire extended a 
foot from the top of the cross, to draw off the electricity from the clouds. 

The kite thus constructed was raised by Franklin and his son in the first 
thunder-storm that occurred in June, 1752. Hempen twine was used, at the 
lower end of which a key was fastened for a prime conductor, while the whole 
was insulated by a silk ribbon fastened to a non-conductor sheltered from the 
wet. With intense anxiety the philosopher awaited the result. A cloud 
passed without any electrical indications, and he began to despair of success. 
Another came, and now to his indescribable joy he saw the loose fibres of 
the twine stand out every way and follow his finger as it passed to and fro. 
Presenting his knuckle to the key, he received a spark ; and as soon as the 
twine wa» wet with rain, and its conducting power thus increased, the elec- 
tricity was abundant. A Leyden jar was charged from the key, with which 
spirits were set on fire, and other experiments performed.— This discovery 
raised its author to the first rank among the philosophers of his day. His 
own feelings at the triumphant result of his experiment may be imagined. 
" Convinced of an immortal name, he felt he could have been content if that 
moment had been his last." 

Franklin's experiment was repeated with success in various parts of Eu- 
rope. There was no room left for doubting the identity of lightning with 
the electric spark. In later times this identity has been further confirmed by 
phenomena connected with the electric telegraph. Reports as loud as that 
of a pistol are often heard in telegraph offices during a storm, and to ensure 
the safety of the operators the wires have to be connected by conductors 
with the earth. Even in clear weather it is sometimes found difficult to fix 
the wires on the poles, in consequence of numbness produced in the hands 
by electricity conducted to them by the wires. 

815. Effects of Lightning. — Lightning produces both 
mechanical and chemical effects. Its mechanical effects are 
very powerful. It crushes huge trees, rends off their 
branches, and sometimes tears their trunks into fragments. 

Relate the incidents connected with Franklin's great discovery. "What was the re- 
sult of this experiment as regards the reputation of its author ? As regards his own 
feelings? Where was the experiment repeated ? How has the identity of lightning 
with the electric spark been since confirmed? 815. Mention some of the mechanical 



314 ELECTRICITY. 

When buildings are struck, large masses of masonry are 
displaced; a brick wall more than 12 feet long has been 
carried in one piece to a distance of 15 feet. These effects 
are analogous to the throwing out of the blocks of wood 
from the gable of the Thunder House, as described in 
§ 797. It is only (as shown in that experiment) in the case 
of imperfect conductors, — that is, when obstructions are 
presented to the free passage of electricity, — that these 
effects are produced. 

Lightning is also a powerful chemical agent. It decom- 
poses water and other substances into their elements. It 
sets fire to trees and houses, and melts metallic bodies. 
On the tops of mountains it is not unusual to see the sur- 
face of the hardest rocks perforated with deep cavities 
covered with a vitreous crust, owing to their having been 
struck with lightning. 

816. Lightning Mods. — When a cloud becomes heavily 
charged with electricity, if another cloud in a different 
electrical state is near it, a discbarge takes place between 
the two ; in which case there is no danger. But some- 
times there is no such adjacent cloud, and a flash of light- 
ning darts from the charged cloud to the earth or sea : it 
is then said to strike. In such a case, the air being a bad 
conductor, the electric fluid in its descent follows any bet- 
ter conductor it can find, such as a house, a tree, the mast 
of a ship, or a living animal. Now, if the objects just 
mentioned were perfect conductors, the lightning would 
follow them to the earth without doing any injury ; but 
they all offer some obstruction to its passage, and therefore 
all suffer more or less when struck. 

The tallest objects, reaching nearest to the clouds, are the most likely to 
be struck. It is therefore imprudent to stand on the top of a hill or near a 
tree during a thunder-storm. In the house it is best at such a time not to 
sit near a damp wall, a bell wire, a gilded picture frame, or any metallic sub- 
jects of lightning. Only in what ease are these effects produced ? State some of 
the chemical effects of lightning. 816. When does an electric discharge take place 
between two clouds ? When, between a cloud and the earth ? Why are houses, 
trees, &c, struck? Why do they suffer damage when struck? What objects are 
most likely to be struck ? What positions is it imprudent to take during a thunder- 



LIGHTNING RODS. 



315 



(stance, as the electric fluid is sure to select the best conductor in its path to 
the earth if the house should be struck. 

817. Having proved lightning to be an electric dis- 
charge, Franklin proceeded to devise means for preserving 
buildings from its effects. He thus became the inventor 
of the Lightning Rod, a simple contrivance which has been 
instrumental in saving life and property to an extent that 
can not be estimated. 

The best material for a lightning rod is copper, but iron is cheaper and 
generally preferred. It must extend at least four feet above the building to 
be protected, and terminate above in one or more sharp points, which should 
be tipped with silver or platinum to keep them from rusting, and thus 



losing part of their conducting power. The rod should be 
continuous, and of such size that the fluid may follow it freely 
without danger of melting it, — say three-fourths of an inch 
across. It should be placed as close as possible to the wall 
and fixed securely to it. The lower end should be divided 
into two or more pointed branches, as shown at a, a, a, 
in Fig. 290. These branches should slant away from the 
building, and at least one of them should sink far enough into 
the ground to reach water or soil that is moist. If the build- 



Fig. 290. 



Fig. 291. 




I 



ing is large, and particu- 
larly if it has more than 
one point projecting up- 
ward, it should have sev- 
eral rods, either descend- 
ingdirectly to the ground, 
like c, d, in Fig. 291, or 
connected together by a 
good conductor, and ul- 
timately carried down 
like e, f, g,h. a 

818. The security afforded by lightning rods is twofold. In the first place, 
terminating in points, they generally draw off the electric fluid silently ; and 
secondly, if a discharge takes place, the lightning in its descent will follow 
them rather than the inferior conductors to which they are attached, and 
finding a free passage through them will do no injury. — Lightning rods have 
not been found efficacious to a greater distance than forty feet. Within this 
limit, they protect a space around themselves equal to twice the height that 



storm ? S17. "Who invented the Lightning Eod ? Of what materials is the lightning 
rod made ? "What should he its form and size, to ensure the safety of a building '? 
In what case should a building have several rods ? How may they in that case be 
arranged? SIS. In what two ways do lightning rods conduce to the safety of a build- 
ing? What is the greatest distance at which lightning rods nave been found effica- 



316 ELECTRICITY. 

they project above the building ; for example, a rod projecting five feet wili 
protect every point of the surrounding surface within ten feet of itself. 

819. Electrical Fish. — The torpedo, the Surinam eel, 
the si-lu'-rus electricus, and several other species offish, have 
a peculiar organ with which they can give electric shocks, 
more or less powerful according to their size. They use 
this organ for defending themselves against enemies, and 
for stunning and thus securing their prey. The power of 
giving shocks ceases with life ; its too frequent exercise 
exhausts the fish and ultimately kills it. The shock of a 
torpedo fourteen inches long is borne with difficulty ; and 
the Surinam eel has been found of such size that its shock 
proved immediately fatal. 

The Surinam eel gives as many as twenty shocks a minute, yields the 
electric spark in the air, and charges a Leyden jar. Faraday computed that 
the average shock of one of these eels on which he experimented was equal 
to the discharge of a battery of fifteen jars, containing 3,500 square inches of 
glass, charged as heavily as possible. — The South American Indians catch 
these eels by driving a number of wild horses into a pond containing them. 
The eels, roused from their muddy retreats, vigorously defend themselves 
by pressing against the stomachs of the horses and repeatedly discharging 
their electrical battery. The poor beasts, panting from their struggles, with 
mane erect and haggard eyes expressing fright and anguish, seek to escape 
from their invisible foes, but are driven back by the Indians who surround 
the pond, armed with long reeds, and making terrible outcries. After sev- 
eral of the horses are stunned and drowned the eels become exhausted by 
their continued discharges, and are no longer objects of dread to the Indians. 
Slowly approaching the shore, they are captured with harpoons fastened to 
long cords ; and to such a degree is their electrical power weakened that 
hardly any shock at all is received in drawing them ashore. 

The silurus is a fish twenty feet long, found in the Nile and the Niger ; 
its electrical apparatus lies immediately below the skin and extends round 
the whole body. 

Voltaic Electricity; 

OR, ELECTRICITY PRODUCED BY CHEMICAL ACTION. 

820. Having considered electricity produced by fric- 
tion, we proceed to treat of that developed by chemical 

cious ? Within this limit, how great a space do they protect ? 819. What species of 
fish have the power of giving an electric shock ? For what purposes do they use this 
power ? What is the effect of its too frequent exercise ? What is said of the shock 
of a torpedo fourteen inches long ? Of the Surinam eel ? What was the power of one 
experimented on by Faraday ? How do the South American Indians capture these 



VOLTAIC ELECTRICITY. 317 

action. This branch of the subject is known as Gal- 
vanism. 

821. Galvani's Discovery and Theory. — The first 
discoverer in this department of science was he from whom 
it received its name, Galvani [gal-vah'-ne], Professor of 
Anatomy in the University of Bologna, Italy. The effects 
of atmospheric electricity on the animal frame had long 
engaged his attention. In the year 1790, having prepared 
the hind legs of some frogs suitably for experiment, and 
hung them on copper hooks till they should be needed, he 
observed to his surprise, on accidentally pressing the lower 
extremities against the iron railing of a balcony, that they 
were drawn up with a singular convulsive action. He 
found upon experiment that similar contortions were pro- 
duced whenever copper and iron, connected with each 
other, were brought in contact, the one with the nerves of 
the thigh, the other with the muscles of the leg. 

Galvani's experiment is often repeated at the present day. To perform 
it, separate the lower extremities, of a frog from the rest of the body, skin 
them, and pushing back the muscles on either side of the back-bone, lay bare 
the lumbar nerves. Stretching out the 
legs in the position shown in Fig. 292, 
lay a thin curved rod of zinc under the 
nerves, and touch the muscles of the 
leg with a similar rod of copper. As 
long as the rods are kept apart, there is 
no movement in the legs ; but the in- 
stant they are brought in contact, a vi- 
olent convulsive motion takes place, the 
legs are drawn into the position shown 
by the dotted lines, and these contor- 
tions are repeated as often as the rods 
are separated and again brought to- 
gether. 

Galvani attributed this convulsive 
movement to a certain vital fluid which he supposed to reside in the nerves, 
and to pass to the muscles over the metallic conductors, in a manner similar 
to the passage of electricity between the inner and the outer coating of a 

eels ? What is said of the silurus ? 820. What is Galvanism ? 821. From whom did 
it receive its name ? Give an account of Galvani's discovery. How may GalvanVs 
experiment be repeated at the present day ? When do the contortions take place ? 
To what did Galvani attribute this convulsive movement ? What did he call this 




3] 8 VOLTAIC ELECTRICITY. 

JLeyden jar when it is discharged. He therefore called this supposed fluid 
Animal Electricity ; but in compliment to its discoverer it soon became known 
as Galvanic Electricity, or the Galvanic Fluid. 

822. Volta's Theory axd the Voltaic Pile. — Prof. 
Volta, of Pavia, experimenting further on the subject, soon 
laid aside Galvani's theory of a " vital fluid ", and held that 
the effects in question were caused by the contact of the 
two dissimilar metals ; that the legs of the frog had no 
agency in producing the galvanic excitement, but merely 
gave indications of its presence, like the pith ball electro- 
scope in the case of ordinary electricity. To prove this, he 
combined the metals apart from all animal organizations ; 
and advancing step by step, about the year 1800, he gave 
to the world his celebrated Pile, the appearance of which 
marked a new era in the history of electrical science. 

Volta's "contact theory" was at one time generally received; but it is 
now known that the galvanic excitement is not produced by the mere con- 
tact of the metals, but by chemical action. A third element, such as the 
moisture of tbe hand, animal fluids, an acid, or some saline solution, must 
act chemically on one of the metals. It is believed that no chemical action 
ever takes place without the development of electricity, though it may be 
in so small a degree as to escape our sense*. 

823. Volta's Pile consisted of a number of circular plates 
of copper and zinc, and pieces of cloth moistened with a 
weak acid or saline solution, alternating as follows, the same 
order being observed throughout. At the base of the pile 
was a plate of copper, and on this a zinc plate, the two 
constituting a pair. On this pair was a piece of cloth moist- 
ened as above, then a second similar pair (the copper al- 
ways below), then a piece of cloth, a third pair, and so on 
to the top of the pile. The whole was insulated on glass, 
and a wire was attached to each end. The wire connected 
with the zinc plate at the top of the pile yielded positive 
electricity; that connected with the copper plate at the 
base, negative. When the ends of these wires were brought 



Bupposed vital fluid? What other names were soon given to it ? 822. Who experi- 
mented further on the subject ? State Volta's theory. To what invention did Volta" £ 
investigations lead ? What is now thought of Volta's " contact theory " ? With what 
!d chemical action always accompanied ? 823. Of what did Volta's Pile consist ? De- 



VOLTA'S PILE. 319 

together or separated, a bright spark was produced. A 
very fine platinum wire, half an inch long, stretched between 
the ends of the wires, was made red hot. A person taking 
one of these wires in each hand, received a succession of 
shocks, like those from a Leyden jar, but slighter, — their 
intensity depending on the number of plates. These effects 
were produced as long as the arrangement and condition 
of the plates remained unchanged. 

Volta's pile, immediately connected as it was with the Galvanic Battery 
(which has since superseded it), was one of those inventions to which science 
is most largely indebted. It has immortalized its author, in honor of whom 
this species of excitement produced by chemical action is now generally 
called Voltaic Electricity. 

824. Familiar Experiments. — The effects of voltaic 
electricity may be illustrated with familiar experiments. 

Experiment 1. — Place a piece of zinc under the tongue, and on the tongue 
a silver coin. As long as the metals do not touch, nothing is perceived ; 
but as soon as they are brought in contact, the voltaic circuit is formed, a 
thrilling sensation is felt in the tongue, a taste somewhat like copperas is 
perceived, and, if the eyes are closed, a faint flash of light is seen. Here 
electricity is developed by the chemical action of the saliva upon the zinc. 

Exp. 2. — Lay a silver dollar on a sheet of zinc, and on the coin place a 
living snail or leech. No sooner does the creature in moving about get 
partly off the dollar and on the zinc, than it receives a shock and re. 
coils. In this case it is the slime of the snail or leech that acts chemically on 
the zinc. 

825. Galvanic Batteries. — Soon after inventing the 
pile, Yolta proposed another arrangement for the metallic 
plates, identical in principle, but more convenient for use. 
He discovered that electrical excitement was exhibited 
whenever slips of copper and zinc were immersed in a ves- 
sel containing some diluted acid, if the circuit was com- 
pleted by bringing the metals themselves, or wires con- 
nected with them, in contact above the vessel. Such an 
arrangement is called a Simple Galvanic Circle ; it isi 



scribe some of its effects. How long were these effects produced ? What is said of 
the invention of Volta's Pile ? What is electricity produced by chemical action now 
generally called ? 824. What is the first experiment with which the effects of voltaio 
electricity are familiarly illustrated? The second experiment ? 825. Soon after in- 
Venting the pile, what discovery did Volta make ? What is such an arrangement 



320 



VOLTAIC ELECTRICITY. 



Fig. 294. 



Fig. 293. shown in Fig. 293. Combining a num- 

ber of vessels similarly prepared, Yolta 
made the first galvanic battery, known 
as the Couronne des Tasses [koo-rone' da 
tahs]. 

826. The Couronne des Tasses, or " crown of cups " P 

represented 

in Fig. 294, 

consisted of 

any number 

of vessels, 

each con- 
taining a slip 

of copper 

and zinc, the 

copperofone 

vessel being couronne des tasses. 

connected by a conductor with the zinc of the next. 

To complete the circuit, wires attached to the extreme 

metallic slips of the series were brought together, 
when a spark and other electrical phenomena were produced. 

827. Trough Battery. — Instead of the separate cups used by Volta, one 
long vessel divided into cells was subsequently employed. The zinc and 
copper plates, connected in pairs by a slip of metal, and arranged at such 





SIMPLE GALVANIC 
CIECLE. 



Fig. 295. 




SMEES BATTERY. 



distances as to enclose a partition between the zinc 
and copper of each pair, were fastened to a common 
frame, so that they could all be immersed in acid and 
thus subjected to chemical action at the same time. 
This improved arrangement was known as the Trough 
Battery. 

828. Smee's Battery. — Smee's Battery (see Fig. 295) 
has three metallic plates suspended, without touching 
each other, from a wooden frame. The middle plate is 
of silver coated with platinum. The outside ones are 
of amalgamated zinc, — that is, zinc coated with mer- 
cury. The whole are immersed in dilute sulphuric 
acid contained in an earthenware vessel. No action 
takes place till communication is established between 
the metals, when a bubbling immediately commences 
in the liquid, and voltaic electricity is produced. This 
battery, though not so powerful as those hereafter de- 



Jalled? What name was given to the first galvanic battery, made by Volta? 
826. Describe the Couronne des Tasses. 827. Describe the Trough Battery. 828. De- 
scribe Smee's Battery. What are the advantages of this battery ? For what is it 



daxiell's constant battery. 



321 



scribed, is economical, may be kept in operation for several days, and is mucb 
used in plating tbe inferior metals with gold and silver. With certain mod- 
ifications it has also been employed in working the magnetic telegraph. 

829. In the batteries thus far described but one fluid 
was used, and two metals of such a nature that one was 
more readily acted on by the fluid than the other. Dilute 
sulphuric acid being used as the fluid, zinc (which it readily 
acts upon) was generally taken for one of the metals. 
Great improvements have been made on these single fluid 
batteries. With the exception of Smee's, they have been 
entirely superseded by instruments in which two fluids are 
employed, and which are not only more powerful, but also 
more regular and permanent in their action. The most 
important of these we proceed to describe. 

830. Darnell's Constant Battery. — The two-fluid batteries are Fig. 296. 
all modifications of Daniell's, which was invented in 1836. It con- 
sists of an outer cylinder of copper, within which is a cup of un- 
glazed porcelain, of the shape represented in Fig. 296. Within 
this cup is a solid cylinder of amalgamated zinc. From both the 
zinc and the copper cylinder project brass cups (see Fig. 297) pro- 
vided with screws for the insertion of wires ; the extremities of 
which, if there be but one cell, are called the Poles of the bat- 
tery. If there be several cells, strips of metals inserted in these 
cups connect the zinc of one with the copper of the next, and 
wires for conducting the fluid ai e attached to the zinc of one of 
the extreme cells and the copper of the other. The porous cup 
is filled with dilute sulphuric acid. The copper cylinder is filled 
with the same fluid saturated with sulphate of copper; and ou a 
perforated shelf near its top (represented by the circular dotted 
lines in the figure) is placed some of the solid sulphate, that as 
fast as this substance is used up by the chemical action a fresh 
supply may be obtained, and the operation of the battery thus v *-^lll-- / ' 
made constant. 

As soon as the poles are joined, a powerful action commences, which, 
instead of constantly diminishing as in the single fluid batteries, is main- 
tained for hours without losing any of its efficiency. For ordinary use 
two dozen such cells are combined in a battery. One of the chief im- 
provements in this apparatus is the introduction of the porous cup, which 



^ 



Fig. 297. 




used ? 829. In the batteries thus far described, what are employed for the purpose 
of producing chemical action ? W ich is the most efficient of the single fluid batte- 
ries? How do the single fluid batteries compare with those in which two fluids are 
used? 830. By whom and when was the first two-fluid battery invented ? Describe 
Danieirs Constant Battery, and its mode of operation. What is one of the chief im- 



21 



322 



VOLTAIC ELECTRICITY. 



keeps the liquids apart, yet does not prevent the passage of voltaic cur- 
rents. 

831. Grove's Battery. — Grove's Battery is the most powerful one yet con- 
structed. It operates on the same principle as Daniell's, but employs differ- 
ent metals and fluids, which render it more active. The porous cup contains 
a strip of platinum immersed in strong nitric acid, and is itself contained in 



Fig. 298. 




a zinc cylinder filled with dilute 
sulphuric acid. The whole is 
set in a vessel of glass or earth- 
enware. Fig. 298 shows one of 
Grove's batteries consisting of 
six cells, as arranged by Benja- 
min Pike, jr., of New York. 
The platinum of each cup is con- 
nected with the zinc of the next. 
At the extremities of the cir- 
cuit, wires are attached respec- 
grove's battery. t i ve ly to the platinum of one 

cell and the zinc of the other, the former of which exhibits positive electricity 
and the latter negative. 

Grove's battery is the best for performing the more striking experiments 
of galvanism, being nearly twenty times as powerful as a zinc and copper 
battery containing the same amount of metallic surface. Its superiority is 
owing to the absorption of the hydrogen evolved, the high conducting power 
of the fluids employed, and the ease with which nitric acid is decomposed. 

832. Buv-sen's Battery. — The cost of platinum renders Grove's apparatus 
expensive. Bunsen therefore devised a battery, in which plates of carbon 
acted on by nitric acid are substituted for platinum. In other respects it is 
like Grove's, but it is less efficient. 

833. Dry Piles. — Feeble galvanic currents may be pro- 
duced by compressing a great number of circular pieces of 
copper and zinc paper (sometimes Called gold and silver 
paper), placed back to back, in a varnished glass tube, 
which they exactly fit. As in Volta's pile, the same order 
must be observed throughout. The electrical excitement 
produced by a Dry Pile (as such an apparatus is called) 
lasts a long time. Bells have been kept constantly ringing 
for eight years by the alternate attraction and repulsion of 
a clapper suspended between two such piles. 



provements in this apparatus ? 831. Describe Grove's Battery. How does it com- 
pare in power with a zinc and copper battery ? To what is its superiority owing ? 
832. What is the objection to Grove's battery ? To remove this, what modification 
Aid Bunsen propose ? 833. How are Dry Piles formed? What evidence is adduced 



THEORY OF THE GALVANIC BATTERY. 323 

834. Quantity and Intensity. — The quantity of vol- 
taic electricity produced by a battery, depends on the size 
of the metallic plates employed ; its intensity, on their 
number. 

The difference between the quantity and the intensity of the electric fluid 
is analogous to the difference between the quantity of a solid dissolved in a 
given liquid and the strength of the solution. Into a hogshead of water 
throw a wine-glass full of salt, and into a tea-spoon full of water put as much 
3alt as it will dissolve. The former solution will contain a greater quantity 
of salt than the latter, but it will be less strong. 

835. Theory of the Galvanic Battery. — Let us now 
inquire how electricity' is developed with the galvanic bat- 
tery. Take, as an example, Volta's single fluid apparatus. 
When the zinc and copper plates are immersed in acidu- 
lated water, and connection is established between them, 
the water is decomposed into its elements, oxygen and hy- 
drogen. The oxygen combines with the zinc, for which it 
has a strong affinity, and forms oxide of zinc ; while the 
hydrogen appears about the copper in the form of minute 
bubbles. The zinc, in consequence of the chemical change 
produced in its surface, parts with its positive electricity to 
the liquid, and remains negatively electrified. The copper, 
not acted on by the liquid as the zinc is, attracts from it 
this same electricity, and becomes positively electrified. 
The acid mixed with the water tends to dissolve the oxide 
of zinc as fast as it is formed, and thus to keep a fresh sur- 
face of the metal exposed to the liquid. 

836. The terminal wires of a battery, or, when no wires 
are attached, the plates from which they would proceed, 
are called its Poles. The pole connected with the metal 
most easily acted on by the fluid, always exhibits negative 
electricity; the other, positive. F or pole some substitute 
the term electrode, meaning the path by which a voltaic 
current enters or leaves a body. The positive pole they 

of the permanency of their action ? 834 On what does the quantity of voltaic elec- 
tricity produced by a battery depend ? On what, its intensity ? Illustrate the differ- 
ence between the quantity and the intensity of the electric fluid. 835. Give the the- 
ory of the operation of the galvanic battery. 836. What is meant by the Poles of a 
oattery ? Wbicb pole exhibits negative electricity i "Which, positive ? What term 



324 VOLTAIC ELECTRICITY. 

call the Anode (ascending or entering path) ; the negative, 
the Cathode (descending or departing path). When the 
electrodes are brought in contact, the galvanic circuit is 
said to be closed. The two currents then meet and neu- 
tralize each other ; but, as fresh currents are all the time 
being produced, the action continues without interruption 

837. Difference between Frictional and Voltaic 
Electricity. — Voltaic electricity and that developed by 
friction are the same in kind, but are characterized by cer- 
tain points of difference. 

1. The electricity developed by friction is far more in- 
tense ; that produced by chemical action is far greater in 
quantity. 

A simple galvanic circle (§ 825) develops as much electricity in three sec- 
onds as would be accumulated in a battery of Leyden jars by thirty turns of 
a powerful plate machine. Yet so weak is this voltaic electricity that a per- 
son receiving it through his system would hardly be aware of its passage, 
while the same quantity from the Leyden jars might prove fatal to life. It 
takes a galvanic battery of about fifty pair of plates (no matter what their 
size) to affect a delicate electroscope, and one of nearly a thousand pair to 
make pith balls diverge. 

2. Voltaic electricity will not pass through an insulating 
medium, as the electric spark does. If the circuit is broken, 
all action at once ceases. It will pass thousands of miles 
over a conducting wire, but will not leap a break the fiftieth 
part of an inch. 

3. The chemical effects of voltaic electricity are incom- 
parably greater than those of frictional electricity. 

The galvanic battery produces the most intense heat, and readily decom- 
poses compound substances ; no such effects belong to the electrical machine. 
An ordinary galvanic battery will decompose a grain of water into oxygen 
and hydrogen. To do this with frictional electricity would require the power 
of an electrical plate having a surface of 32 acres, — which would be equiva- 
lent to a flash of lightning. 

838. Effects of Voltaic Electricity. — Among: the 



is by some substituted for pole t What is the Anode ? What is the Cathode ? When 
Is the galvanic circuit said to be closed f What then takes place ? 837. What is the 
first point of difference between frictional and voltaic electricity ? State some facts 
illustrating this difference. What is the second point of difference between frictional 
and voltaic electricity ? The third point of difference ? What facts are stated in the 



DECOMPOSING EFFECTS. 325 

effects of voltaic electricity on substances brought within 
the circuit, may be mentioned the following : — 

839. Decomposition. — Compound substances may be 
decomposed into their elements with the galvanic battery ; 
uid it is a singular fact, that of the elements so obtained 
some always arrange themselves about the positive pole, and 
others about the negative. Thus, oxygen, chlorine, iodine, 
and the acids, invariably fly to the positive pole, when set 
free from any compound substance ; hydrogen, the oxides, 
and the alkalies, to the negative. As the elements must be 
in an opposite electrical state to the poles that attract them, 
we conclude that oxygen, chlorine, &c, are naturally neg- 
ative, — and hydrogen, the oxides, and alkalies, positive. 
Every chemical compound seems to consist of a positive 
and a negative element, held together by electrical at- 
traction. 

The great discovery that water could be decomposed by voltaic electricity 
was made in 1800, immediately after the announcement of Volta's pile, by 
;in experimenter, who observed that gas bubbles rose when the termiual 
wires were immersed in water. Several years later, Davy, after a long course 
of experiments, decomposed the earths and alkalies, which had before been 
universally regarded as simple substances, and thus brought to light a num- 
ber of new metals, the existence of which had not even been suspected. 

840. The decomposition of water is effected with Fig. 

the apparatus represented in Fig. 299. A large glass 
goblet has a frame fitted to its rim, from which are 
suspended two small receivers for the purpose of col- 
lecting the two gases evolved. As water consists of 
2 volumes of hydrogen to 1 of oxygen, one of the 
receivers should be twice as large as the other. Two 
holes in the bottom of the vessel, to which screw 
cups are attached, admit the electrodes from a bat- 
tery, and terminate on the inside in strips of plat- 
inum, which enter the receiver. The vessel being 
filled with water and the battery set in operation, de- 
composition at once commences. Oxygen passes to the positive electrouV 

text to Illustrate this difference ? S39. What is the first effect of voltaic electricity ? 
What singular fact is stated respecting the elements thus obtained ? What elements 
go to the positive pole ? What, to the negative ? What is inferred from this fact f 
When and under -what circumstances was it discovered that water could be decom- 
posed by voltaic electricity ? What great discovery was made by Davy ? S40. De- 
scribe the mode of decomposing water with the galvanic battery. How is the process 





326 VOLTAIC ELECTRICITY. 

(which should be inserted in the smaller receiver) and hydrogen to the nega- 
tive. The identity of the gases may be proved by subsequently experiment 
ing on them. As water is not a very good conductor of voltaic electricity, 
the process is facilitated by the addition of a little sulphuric acid. 

Fig. 800. 841. The decomposition of a neutral salt may be performed 

with the apparatus represented in Fig. 300. A glass tube 
shaped like a V is fitted at each end with a cork and screw. 
Through these screws pass the wires from a battery, termi- 
nating inside in platinum strips. The tube having been filled 
with a solution of sulphate of soda or any Other neutral salt, 
colored blue with tincture of violets, the battery is set in ac- 
tion. No sooner is a current passed from pole to pole through 
the liquid, than the latter is decomposed. The acid passes to the positive 
pole, and the alkali to the negative. This is shown by the change of color 
produced, the liquid becoming red around the positive wire and green around 
the negative. If tho poles be transposed, the effects will be reversed. 

842. The decomposing power of the galvanic battery is 
turned to practical account in the various processes of 
Electro-metallurgy. This is the art of depositing on 
any substance a coating of metal from a metallic solution 
decomposed by voltaic electricity. One of the branches 
of this art is Plating, which consists in covering the inferior 
metals with a thin coat of gold or silver. When the metal 
coating is not to adhere permanently to the surface on 
which it is deposited, but to form a copy of it and be re- 
moved, the process is called Electrotyping. 

The different processes of Electro-metallurgy differ 
somewhat in their details and in the apparatus employed, 
but the principle involved is the same in all ; viz., that any 
compound metallic solution is decomposed by the passage 
through it of a voltaic current ; whereupon the pure metal 
is attracted to the negative pole, while the substance be- 
fore combined with it goes to the positive. A medal, an 
engraving, or any conducting substance, has therefore only 
to be attached to the negative pole, and the metal in ques- 
tion will be deposited on it, the thickness of the coat de- 

facilitate.d ? 841. With what apparatus, and how, may a neutral salt be decomposed? 
842. How is the decomposing power of the galvanic battery turned to practical ac» 
count? What is Electro-metallurgy ? In what does Plating consist? In what, 
Electrotyping ? What is the principle involved m all the processes of electro-metal- 
lurgy ? When any conducting substance is attached to the negative pole, what take* 






ELECTROTYPING. 327 

pending on the length of time it is left to the action of the 
battery. 

Reversed copies are thus obtained; the minutest indentations on the sur- 
face of the original being represented by elevations on the copy, and projec- 
tions on the original by corresponding indentations in the copy. If an exact 
and not a reversed copy is wanted, a mould, taken from the original in wax 
or plaster, must be submitted to the above process. 

This metallic deposit will take place only on a good conductor ; if, there- 
fore, the object to be copied is not such, it must be endowed witb conducting 
power by dusting over it some fine plumbago. On the contrary, if there is 
any part of which a copy is not wanted, it may be covered with varnish 
which is a non-conductor. — That the copy may be readily removed from the 
original, the surface of the latter should be rubbed with oil or powdered 
plumbago. 

843. The most convenient mode of electro typing is as follows : — Fill a 
trough with a solution of sulphate of copper, and over its top extend two par- 
allel rods of wood a short distance apart. Run the positive wire from a bat- 
tery along one of these rods, and the negative along the other. From the 
negative wire suspend in the fluid the object to be copied, and from the posi- 
tive one a piece of copper plate. Sulphate of copper is composed of sulphu- 
ric acid and copper. When the battery begins to operate, this fluid is de- 
composed ; the copper is drawn to the negative pole and deposited on the 
object attached to it. The sulphuric acid goes to the copper plate, and 
combining with it forms sulphate of copper, thus providing fresh metallic 
solution as fast as the original supply is used up. 

844. Much use is made of the electrotype process. It has to a certain ex- 
tent taken the place of stereotyping in the preparation of plates from which 
books, charts, maps, &c, are printed. Copperplates being harder than those 
of type-metal, a far greater number of copies can be printed from them, and 
they are therefore preferable for works that are likely to have an extensive 
circulation. When the types are set, a mould of each page is taken in wax, 
brushed over with plumbago, and subjected to the above process till a thin 
deposit is formed, which is made of sufficient thickness to print from by back- 
ing it with type-metal. This book is printed from electrotype plates. 

Engravings both on wood and copper are reproduced in the same way, 
their fine lines being brought out with exquisite perfection. The originals 
are put away, and the duplicates alone used in printing. By multiplying 
copies, which is done with little or no injury to the face of the original, any 
number of impressions can be obtained. — Fac-similes of delicate leaves, the 
wiftgs of insects, and even daguerreotypes, may be made in a similar way. 

place ? What sort of copies are thus obtained ? What must be done, to obtain fac- 
similes ? On what alone will this metallic deposit take place ? How may it be made 
to take place on a bad conductor ? What precaution is necessary, to enable us to re- 
move the copy from the original ? 843. Describe the most convenient mode of elec- 
trotyping. 844. For what is the electrotype process used ? In what case are copper 
plates preferable to those of type-metal ? State the process gone through in pre- 



328 VOLTAIC ELECTRICITY. 

845. Protection of Metals. — Voltaic electricity has been 
applied to the protection of metallic surfaces from corro- 
sion. If a given metal is acted on by an acid or saline so- 
lution, we have only to immerse in the liquid some other 
metal more readily acted on by it, and close the circuit by 
connecting the two, when the chemical action on the for- 
mer metal at once ceases and is transferred to the latter. 

Davy proposed on this principle to protect the copper sheathing on the 
bottom of vessels from the action of sea-water. Strips of zinc were fastened 
at certain distances on the copper, and it was found that the latter metal was 
thus perfectly preserved from corrosion. No practical use, however, could 
be made of this proposed improvement ; for shell-fish, sea- weed, &c, which 
had before been kept off by the poisonous properties of the corroded copper, 
now adhered to the bottom in such quantities as to make the vessel sail more 
slowly. 

846. Luminous and Heating Effects. — When the gal- 
vanic circle is closed or broken, — that is, when the two 
terminal wires are brought in contact or separated, — a 
bright spark passes between them. With the proper ap- 
paratus, this spark may be intensified into the most bril- 
liant light yet produced by art, known as the Electric 
Light, or the Voltaic Arch. 

To produce the electric light, connect the poles of a 
powerful battery with the rods of a universal discharger 
(§ 780), and to the extremities of these rods fix charcoal 
points, or pieces of graphite pointed like a pencil. The 
battery being set in operation, the charcoal points are 
brought in contact, and then gradually withdrawn from 
each other a short distance, when the space between them 
is spanned by an arch of intensely bright light. 

The voltaic arch is widest in the centre; its length varies with the power 
of the battery, ranging between three-fourths of an inch and four inches. 
No luminous appearance is produced unless the points first touch, no matter 
how close together they are brought, the air between being an insulator and 



paring the plates. What else are reproduced by the electrotype process? 845. To 
what has voltaic electricity been applied ? How may a metal acted on by a liquid in 
which it is immersed be protected from corrosion ? What application of this princi- 
ple was proposed by Davy ? What was the result of the experiment ? 846. What 
takes place when the galvanic circuit is closed or broken ? Into what may this spark 
be intensified ? How is the electric light produced ? What is the shape of the arch. 



THE ELECTRIC LIGHT. 329 

breaking the circuit. In a vacuum, however, the arch may be formed with- 
out previous contact ; and even in the air, if, -when the points are brought 
near each other, a charge from a Le yden jar is passed from one to the other. 
The voltaic arch, like the electric spark, is entirely independent of com- 
bustion. None of the carbon is consumed, though a portion of it is mechan- 
ically carried over with a sort of hissing sound from the positive to the 
negative electrode, as is shown by the change of shape in the points when 
the experiment is over. The electric light may be produced in a vacuum 
and even under water, which shows that it is not the result of combustion. 

It was long sought to utilize the electric light for 
illuminating purposes, but without success, on account of 
the expense of maintaining the voltaic current. Magneto- 
electric machines (§ 929), however, have been found to 
furnish the necessary electric energy at much less cost. 
Parts of Paris, St. Petersburg, and other cities, are now 
illumined by the electric light ; and workshops, steamers, 
public gardens, etc., are introducing it. 

847. Intense heat, as well as light, is produced by the 
galvanic battery. The most refractory substances — 
quartz, the earths, the precious stones — introduced within 
the voltaic arch, or between the electrodes of a powerful 
battery so as to close the circuit, are instantly fused. 
Thin leaves of the metals yield flames of different colors. 
Gold and zinc burn with a vivid white light, silver with 
an emerald green, copper and tin with a pale blue, lead 
with a bright purple, and steel watch-spring with dazzling 
scintillations. Platinum, which withstands the fiercest 
heat of the furnace, melts like wax ; before doing so, 
however, it becomes incandescent and extremely brilliant. 
This fact has been turned to account by Mr. Edison in 
his form of the electric light, for a description of which 
see Appendix, p. 455. 

The heating power of a galvanic battery may be shown by experiments 
with wires of different metals stretched between the electrodes. A wire so 



and its length ? What is essential to its production in the air ? Is this necessary in 
a vacuum ? How is it proved that the electric light is not the result of combustion ? 
What practical application has recently been made of the electric light ? 847. What is 
said of the heat produced by the galvanic battery ? State some of its effects on the 
metals, etc. What is its effect on platinum ? What use has been made of this fact by 



330 VOLTAIC ELECTRICITY. 

placed instantly becomes hot ; if not too long, red hot. By reducing its length, 
we may raise it to a white heat, and by shortening it still further we may 
fuse or ignite it. Experiments with different metallic wires of the same size 
and length, show that they are not all heated to the same degree by a given 
battery. The best conductors allow the current to pass with the least ob- 
struction, and are therefore heated the least. 

Platinum wire (which is one of the poorest metallic conductors and there 
fore most readily heated), immersed in a small quantity of water between the 
electrodes of a battery, causes the water to boil. Passed through phospho 
rus, ether, and alcohol, it ignites them. Gunpowder is exploded by contac 
with such a wire, a fact which is turned to account in the firing of blasts and 
submarine batteries. The platinum wire being carried through the powder 
and connected with the positive and negative electrodes, no matter how fa: 
off the battery may be, the moment the circuit is completed the platinum be- 
comes red hot, and the explosion takes place. By thus simultaneously firing 
a number of charges of powder placed in deep holes at certain distances, 
600,000 tons of rock have been instantly blown off from the face of a cliff, 
with an immense saving of labor, and with perfect safety on the part of the 
operator, who with his instrument was a fifth of a mile from the scene of 
the blast. 

848. Physiological Effects. — The singular effects of the 
galvanic fluid on the nerves and muscles of animals, origi- 
nally led, as we have seen, to the development of the sci- 
ence of Galvanism, and were carefully investigated in the 
earlier stages of its history. The more powerful instru- 
ments since invented have enabled experimenters to push 
their researches still further. 

When we grasp the electrodes of a battery of fifty cups, 
one in each hand, we feel a peculiar twinge in the elbow 
and sometimes in the shoulder, as if the joints were being 
wrenched apart. This sensation continues as long as the 
electrodes are held in the hands, and when we first grasp 
them or let them go is sufficiently sudden and vivid to be 
called a shock. A number of persons may take the shock 
at once by joining their hands, which should be previously 



Edison? What is the effect of the galvanic battery on metallic wires ? When wire* 
of different metals are used, what is found ? How is this explained ? What expert- 
ments may be performed with a platinum wire fixed between the electrodes of a bat- 
tery ? Describe the process of firing a blast with such a wire. What instance is 
mentioned of the practical application of this process ? 848. What originally led to 
the development of galvanism as a science ? What sensation is experienced on grasp- 
ing the electrodes of a battery ? How may a number of persons take the shock 1 



PHYSIOLOGICAL EFFECTS. 331 

moistened. A weak current passed through the eyes pro- 
duces a faint flash ; passed through the ears, a roaring 
sound ; and through the tongue, a metallic taste. 

The effects of the galvanic battery on the animal system, unlike its lumi- 
nous and heating effects, are found to depend on the number of plates em- 
ployed rather than their size, — that is, on the intensity of the electricity pro- 
duced, and not its quantity. A battery of several hundred pair of plates 
proves fatal to life. One of a hundred pair gives a shock that few would 
like to bear a second time, though, if the plates are small, it has no effect on 
wires stretched between the electrodes. Put the same amount of metallic 
surface in a few pair of very large plates, and such a battery will instantly 
fuse wires subjected to its action, while its shock will hardly be felt. 

849. There seems to be a remarkable analogy between 
a voltaic current and the nervous energy. Experiment has 
shown that, if a nerve be divided, a galvanic current di- 
rected through the region in which it runs will in a meas- 
ure supply its place. The part, which would otherwise be 
palsied from a want of nervous energy, may thus be re- 
stored to its usual action. If, for example, the nerves of 
the stomach are divided, digestion ceases ; but it is resumed 
if the stomach is subjected to galvanic influence. Galvan- 
ism is therefore medically applied in asthma, paralysis, and 
other diseases arising from a prostration of the nervous 
system. 

850. Among the most remarkable effects of voltaic elec- 
tricity are the violent contortions it produces in bodies just 
deprived of life. 

A few years ago, the body of a murderer hanged in Glasgow was sub- 
jected, about an hour and a quarter after his execution, to the action of a 
battery consisting of 270 pair of four-inch plates. One pole was applied to 
the spinal marrow at the nape of the neck, and the other to the sciatic nerve 
in the left hip, when the whole body was thrown into a violent tremor as if 
shivering with cold. On removing the wire from the sciatic nerve to a nerve 
in the heel, the leg was thrown out so violently as nearly to overturn one of 



What is the effect of passing a weak current through the eyes ? Through the ears? 
Through the tongue ? On what do the effects of the galvanic battery on the animal 
system depend ? Compare the different effects of a given amount of metallic surface, 
when thrown into many small plates, and a few large ones. 849. To what does the 
voltaic current bear a remarkable analogy ? What has been shown by experiment ? 
Give an example. In what diseases is galvanism medically applied ? 850. What is 
one of the most remarkable effects of voltaic electricity? Describe the experiments 



332 THERMO-ELECTRICITY. 

the assistants, who tried in vain to prevent its extension. On directing f% 
current to the principal muscle of respiration, the chest heaved and fell, and 
labored breathing commenced. When one of the poles was applied to a 
nerve under the eyebrow and the other to the heel, the most extraordinary 
grimaces were produced : " every muscle of the countenance was simulta- 
neously thrown into fearful action ; rage, horror, despair, anguish, and 
ghastly smiles, united their hideous expression in the murderer's face." Sev- 
eral spectators were so overcome by the sight that they had to leave the 
room, and one gentleman fainted. In the last experiment, the fore finger, 
which had previously been bent, was instantly extended, and shaking vio- 
lently, with a convulsive movement of the whole arm, seemed to point to the 
persons present, some of whom thought that the body had really returned 
to life. 

Thermo-electricity ; 

OR, ELECTRICITY DEVELOPED BY HEAT. 

851. How produced. — If two strips of metals which 
differ in their conducting power, are soldered together at 
one end so as to form an acute angle with each other, and 
heat is applied at the place of junction, a current of elec- 
tricity is produced, which may be carried off by any good 
conductor. Antimony and bismuth exhibit this phenome- 
non in its greatest perfection, and are generally used in 
performing the experiment. Electricity thus developed by 
heat is known as Thermo-electricity. Its properties are the 
same as those of frictional electricity. 

Fig. 301. 852. Thermo-electric Batteries. 

h b b b — Thermo-electricity may be developed 

\f\f\/\/KH abundantly by combining a number of 

V V \l V v thin bars of antimony and bismuth, or 

platinum and iron. They may be ar- 

b b b ranged in either of the forms represent- 

I ed in Fig. 301, or may be laid flat one 

L=U LJ UJ k=D U p 0n another, with pasteboard between 

to prevent them from touching except 

THERMO-ELECTRIC BAT- . . . . 

tekies. at their extremities. By heating the 

points of junction at one end, a, a, a, a, and cooling those 

performed on the body of a murderer shortly after his execution. 851. What is 
Thermo-electricity ? How is it produced ? What metals are generally used in pro- 
ducing it ? 852. How may thermo-electricity be developed abundantly ? How is a 



THERMO-ELECTRIC BATTERIES. 333 

at the other, b, b, b, 6, an electric current is produced, the 
intensity of which is equal to the sum of the intensities of 
the separate pairs. With a wire attached to the first bar 
of bismuth and another attached to the last bar of anti- 
mony, the thermo-electric current may be conducted wher- 
ever it is desired. 

When thirty or forty such combinations are needed, thin metallic bars 
ore used, connected alternately at their extremities, and arranged for conve- 
nience' sake in parallel piles of five or six each. Such a battery indicates 
changes of temperature at its junctions so minute that they can be detected 
in no other way, — even to the hundredth part of a degree of the thermometer. 
The heat radiated from the hand is sufficient to produce a slight electric 
current. 

853. Electricity, besides being produced by friction, 
chemical action, and heat, is also developed under certain 
conditions by magnetism. When so produced, it is called 
Magneto-electricity. This branch of the subject can not 
be understood till we have treated of Magnetism, and will 
therefore be considered in the next chapter, which is de- 
voted to that subject. 



CHAPTER XVII. 
MAGNETISM. 

854. A Magnet is a body which has the property of 
attracting iron and being attracted by it. 

855. Magnetism is the science that treats of the laws, 
properties, and phenomena of magnets. 

Kinds of Magnets. 

856. There are two kinds of magnets, Natural and Ar- 
tificial. 

thermo-electric battery formed ? What is the usual arrangement when a large num- 
ber of such combinations are needed ? How minute changes of temperature are indi- 
jated with such a battery ? 853. By what other agency is electricity also developed ? 
What is it then called? 



334 MAGNETISM. 

857. Natural Magnets. — The natural magnet, or load- 
stone, is an ore of iron, found in great quantities in differ- 
ent parts of the earth, which has the property of drawing 
to itself iron filings, needles, or small pieces of unmagnetic 
iron. Its texture is hard, and its color varies from reddish- 
brown to grey. Besides the loadstone, nickel, cobalt, and 
brass when hammered are found to have magnetic proper- 
ties, though in an inferior degree. 

858. The attraction of the loadstone for particles of iron appears to have 
been known to the Greeks, Chinese, and other nations in remote antiquity. 
It is distinctly alluded to by Homer and Aristotle. Pliny speaks of a chain 
of iron rings suspended one from another, the first of which was upheld by 
a loadstone. He tells us, also, that Ptolemy Philadelphus proposed to build 
a temple at Alexandria, the ceiling of which was to be of loadstone, that its 
attraction might hold an iron statue of his queen Ar-sin'-o-e suspended in the 
air. Death prevented Ptolemy from carrying out his design ; but St. Au- 
gustine, at a later day, mentions a statue thus actually held in suspension in 
the temple of Se-ra'-pis, at Alexandria. — The magnet (magnes in Greek) is 
supposed to have received its name from Magnesia, a city of Asia Minor, near 
which it was first found. 

859. Poles. — The attractive power of a natural magnet 
does not reside equally in all its parts, but is strongest at 
its extremities and diminishes towards the middle, where 
it is entirely wanting. This is shown by rolling a piece of 
loadstone in iron filings. They will be found to cluster 
about the ends, those that first adhere being endowed with 
the power of attracting others, till large tufts are formed, 
while the middle is left entirely bare. 

The points at which the greatest attractive power is 
exhibited, are called the Poles of the magnet. The central 
part, where it is wanting, is called the Neutral Line. 

If a piece of loadstone is broken, each portion becomes 
a perfect magnet, and has poles of its own. 

854. What is a Magnet ? 855. What is Magnetism ? 856. How many kinds of 
magnets are there ? Name them. 857. What is the natural magnet ? What other 
metals have magnetic properties ? 858. To whom and when was the attraction of 
loadstone for iron known ? What ancient authors allude to it ? Of what does Pliny 
speak ? What use did Ptolemy Philadelphus propose to make of the loadstone ? 
What is mentioned by St. Augustine ? From what did the magnet receive its name ? 
859. What is shown by rolling a piece of loadstone in iron filings ? What is meant 
by the Poles of the magnet ? What is the Neutral Line ? If a piece of loadstone is 



NATURAL MAGNETS. 



335 



860. Power of Natural Magnets. — "When quite small, a 
natural magnet will sustain many times its own weight of 
iron. Sir Isaac Newton is said to have worn, in a ring, a 
piece of loadstone weighing three grains, which would lift 
750 grains of iron. Their attractive power, however, does 
not increase with their size. Large pieces of loadstone 
never support more than five or six times their own weight, 
and rarely as much. The most powerful natural magnet 
known is capable of lifting 310 pounds. 

861. Armature. — The power of 
a natural magnet is increased by 
applying vertically to its opposite 
polar surfaces thin strips of soft 
iron, projecting a little below, and 



Fig. 302. 



bent, as shown in ap, b 




Fig. 



fe 



R 



Fig. 
302. The attractive force then 
centres in p and n, which become 
the new poles. This arrangement is called an Armature, 
and a magnet so prepared is said to be armed. 

To keep the armature in its place, metallic 
bands, A B, C D (Fig. 303), are passed round the 
whole. A ring, R, is attached to the top for con- 
venience of handling. The effect of the magnet 
is further increased by uniting its poles with a 
transverse piece of soft iron, K, called the Keeper. 
To this a hook is attached for suspending a scale- 
pan and weights. 

862. Artificial Magnets. — A piece 
of iron or steel brought in contact with 
a natural magnet or very near it, ac- 
quires its peculiar properties, and will 
itself attract iron filings, needles, &c. 
Soft iron loses these properties on be- 
ing withdrawn from the magnet ; but a piece of steel re- 
tains them permanently, nor does the natural magnet from 





ABMED MAGNET. 



broken, what is said of the fragments ? 860. What is the power of natural magnets, 
when small ? When large ? How much is the most powerful natural magnet known 
capable of lifting ? 861. How is the power of a natural magnet increased? Describe 
the armature and the arrangement for securing it in its place. How is the effect of 



336 



MAGNETISM. 



which it receives them suffer any diminution of power in 
consequence. 

Apiece of iron or steel to which magnetic properties 
have been imparted in any w T ay, is called an Artificial 
Magnet. 

863. Kinds of Artificial Magnets. — There are several 
kinds of artificial magnets, called from their shape Bar Mag- 
nets, Horse-shoe Magnets, and Magnetic Needles. The first 
two are most powerful when formed of several similar pieces 
riveted together, in which case they are called Compound 



Magnets. 







Fig. 304. 




fas 


B jJH 




B Q *=A 








_. _ " ; 








BBi^=^- — -=-- » 



Fig. 305. 




HOE8E-8HOE 
MAGNET. 



COMPOUND BAR MAGNET. 

Fig. 304 represents a Compound Bar Magnet ; Fig. 305, 
a Compound Horse-shoe Magnet. N, S, represent the poles. 
The Horse-shoe magnet has an armature, A, attached, which 
increases and preserves its power, and should always be 
kept on when the magnet is not in use, 

Magnetic Needles are very light magnetic 
bars (see Fig. 306), poised at their centre on a 
pivot, on which they move freely, either hori- 
zontally or up and down. 
S In the former case, they 
■■ are called Horizontal Nee- 

dles; in the latter, Verti- 
cal or Dipping Needles. 

864. Artificial magnets are more 

efficient and regular in their action 

than natural ones, and are therefore 

preferred for purposes of experi- 

magnetio needle. merit. The horse-shoe is more 



N 



Fig. 306. 
A 



.he magnet further increased? 862. How may magnetic properties be imparted to a 
piece of iron or steel ? What is the difference between soft iron and steel in this 
connection ? What is an Artificial Magnet ? 863. Name the different kinds of artifi- 
cial magnets. What are Compound Magnets? What do Figs. 304, 305, represent ? 
What are Magnetic Needles ? Into what two classes are they divided ? 864. How 
\o artificial magnets compare in efficiency with natural ones ? How does the horse- 



PROPERTIES OF THE MAGNET. 337 

powerful than the bar magnet. A horse-shoe of one pound has been known 
to sustain 26 1 / 2 pounds. 

865. Poles. — The poles of an artificial magnet, — that is, 
the points in which the greatest attractive force resides, — ■ 
are found to be about one-tenth of an inch from the ex- 
tremities. In very long bar magnets, besides the two poles 
always situated near the extremities, two other poles, nearer 
the centre, are sometimes, though rarely, found. 

866. The power of a magnet, whether natural or artifi- 
cial, may be increased by daily adding a little to the weight 
which it will support. If, for instance, a given magnet just 
sustains two pounds of iron, by putting on a small addi- 
tional weight every day, we may perhaps make it sustain 
three or even four pounds. If, on the other hand, we over- 
load it, so that the armature falls off, the power of the mag- 
net will be impaired. Any rough treatment, such as ham- 
mering the magnet, rubbing it violently, or letting it fall, 
has the same effect. Heat, also, diminishes the power of a 
magnet. Red heat destroys it altogether, even after the 
magnet has cooled. 

867. Air is not essential to the action of a magnet; all 
its phenomena may be exhibited in a vacuum. 

Properties of the Magnet. 

868. Attraction. — As stated above, all magnets attract 
unmagnetic iron. They are also attracted by it. 

Suspend a magnetic needle by a thread. Bring a piece of iron near either 
extremity, and the needle will be drawn towards it. 

869. Magnetic attraction acts with undiminished power 
through any thin substance. 

In the last experiment interpose a piece of glass or paste-board between 
the iron and the needle ; the latter will be attracted none the less. 

shoe compare with the har magnet ? 865. Where do the poles of an artificial magnet 
iie ? What are sometimes found in very long bar magnets ? 866. What is the effect 
of adding a little daily to the weight which a magnet supports ? Give an example. 
What is the effect of overloading a magnet ? Of treating it roughly ? Of heating it ? 
867. Is air essential to the action of a magnet ? Prove it. 868. What is the first prop- 
srty of magnets? What experiment shows the attraction of iron for a magnet? 
869. What is the effect of interposing any thin substance ? How may this fact be 

22 



JJ38 



MAGNETISM. 




MAGNETIC CURVES. 



FlS- 307. Hold a piece of paper over a 

bar magnet, and dust on it some 
iron filings. Under the influence 
of the magnetic attraction trans- 
mitted through the paper, they 
will arrange themselves in regu- 
lar lines, as shown in Fig. 307. 
These lines are called Magnetic 
Curves. — The superior attractive 
power of the poles is also shown 
t>y this experiment ; for the filings are thickest directly over those points, the 
curves appearing to converge there from all directions. 

Magnetic figures of any description may be formed on a steel plate by 
marking on it with one of the poles of a bar magnet, and then sprinkling 
iron filings on the surface. They will at once adhere to the lines which the 
magnet has traced. The result is the same if paper is laid on the steel sur- 
face before the bar is drawn over it, the magnetic influence being transmitted 
through the paper. 

870. Law. — Magnetic attraction decreases in intensity 
as the square of the distance from the magnet increases. 

If two similar substances are situated respectively 1 inch and 2 inches 
from a given magnet, the former will be attracted 4 times as strongly as the 
latter. This law corresponds with that of gravitation, light, and heat. 

871. Polarity. — A magnetic needle, left free to move, 
always points north and south, or nearly so. Often as it 
may be disturbed from its natural position, it invariably re- 
sumes it after a few vibrations. This property is called 
Magnetic or Directive Polarity. 

It is to be observed in connection with magnetic polarity 
that the same e itremity of the needle always points to the 
north, and the same extremity to the south. That which 
points north is called the North Pole ; and that which points 
south, the South Pole. Turn the needle round till its north 
pole points f outh, and it will not rest till it has traversed 
a semicircle and got round again to the north. 

872. If the poles of a bar or horse-shoe magnet be pre- 
sented successively to the north pole of a magnetic needle, 

illustrated ? How are Magnetic Curves formed ? What does this experiment show ? 
How may magnetic figures he formed ? What is the effect of interposing paper be- 
tween the magnet and the steel surface? 870. What is the law of magnetic attrac- 
tion? Give an example. 871. What is meant by Magnetic or Directive Polarity? 
What is to he observed in connection with magnetic polarity ? What name is given 



MAGNETIC POLARITY. 339 

one of them will be found to attract it and the other to 
repel it. If the experiment be tried with a number of dif- 
ferent needles, the same pole will always be found to at- 
tract, and the same to repel. This shows that the two poles 
of the magnet have different properties, which we indicate 
by giving them different names. The one that attracts the 
north pole of the needle we call the South Pole of the 
magnet, and the one that repels it, the North Pole. 

873. General Law. — Like poles of magnets repel each 
other, and unlike poles attract each other. This law corre- 
sponds with that of electrical attraction and repulsion. 

Balance a bar magnet with weights on a pair of scales. Beneath its pos- 
itive pole bring the positive pole of another magnet, and the scale containing 
the bar will rise owing to the repulsion of the like poles. Substitute the neg- 
ative pole, and the scale will descend owing to the attraction of the unlike poles. 

874. Like poles neutralize each other's attraction for 
unmagnetic iron. 

Immerse the positive poles of two magnets separately in iron filings. On 
withdrawing them, both will be covered with large tufts. Now bring them 
together, and the filings will immediately drop off" from both. The result 
will be the same if the experiment be tried with the negative poles of two 
magnets. If the positive pole of one magnet and the negative of the other 
be used, the filings, instead of falling off, will join in a festoon between the 
two unlike poles. 

875. The Astatic Needle.— The 
polarity of two needles of equal 
power may be neutralized by sup- 
porting them on the same pivot, one 
above the other, parallel and with 
unlike poles pointing in the same 
direction. An instrument so formed 
is called the Astatic Needle. 

Fig. 308 represents an astatic needle. The 
north pole of the upper one points the same 
way as the south pole of the under one, and astatic needle. 

to the two poles of the needle ? 872. How is it shown that the poles of a bar magnet 
have different properties ? How are these poles distinguished ? 873. What is the 
law of magnetic attraction and repulsion ? Illustrate this law with an experiment. 
874. What is the effect of like poles on each other's attraction ? Show this experi- 
mentally. 875. How may the polarity of two needles of equal power be destroyed? 




340 MAGNETISM. 

vice versa. The consequence is that the polarity of both is destroyed ; the 
needles will remain in whatever direction they are placed. 

876. When a magnet is divided, each portion becomes 
a perfect magnet in itself, and has its own poles, even though 
the parts in which the new poles lie exhibited no magnetic 
attraction at all before the division. Those extremities of 
the divided portions which lie towards the north pole of 
the original magnet will all be north poles, and the extrem- 
ities towards its south pole will all be south poles. 

877. Magnetic Variation. — In a given place, all mag- 
netic needles point in the same direction. This direction 
is called the Magnetic Meridian. 

In some parts of the earth the magnetic meridian runs 
due north and south ; that is, a plane extended in the di- 
rection in which the needle stands would pass through the 
north and the south pole of the earth. The magnetic me- 
ridian would then correspond with the geographical me- 
ridian. In most places, however, the magnetic meridian 
deviates more or less from the geographical meridian. This 
deviation is called the Variation of the Needle, or Magnetic 
Variation. 

The variation of the needle is different at different places on the earth's 
surface, and is constantly changing at the same place. Recorded observa- 
tions in the old world show that for a series of years the needle kept varying 
more and more towards the west ; till, having attained its western limit, it 
turned back towards the east, in which direction it is now moving. The 
cause of this periodical change and the law which regulates it are as yet un- 
known. At Washington City the variation is now (1879) 3 degrees 47 
minutes west; that is, the needle points 3 degrees 47 minutes west of 
north. Every year it becomes somewhat greater, the annual rate of in- 
crease being about Si minutes. 

Two irregular lines (which are constantly changing) may be traced on 
the earth's surface, one in each hemisphere, along which the needle points 
due north and south. They are called Lines of no Variation. 

878. Magnetic Dip. — An ordinary steel needle, poised 

Describe the Astatic Needle. 876. When a magnet is divided, what is said of each 
portion ? Which extremities of the divided portions will be north poles, and which 
south ? 877. What is the Magnetic Meridian ? In some parts of the earth how does 
the magnetic meridian run ? How, in others ? What is meant by Magnetic Varia- 
tion ? What do recorded observations show? What is the present variation at 
Washington City, and how is it changing from year to year ? What is meant by 



MAGNETIC DIP. 



341 



Fig. 309. 



on its centre of gravity so as to move freely up and down, 
remains in any position in which it may be placed ; if mag- 
netized, in most parts of the earth it inclines more or less 
towards the horizon. This inclination is called the Dip of 
the Needle, or Magnetic Dip. It was discovered in 1576, 
by an optician of London. 

With the Dipping Needle and graduat- 
ed scale attached, represented in Fig. 309, 
the magnetic dip at any given place can be 
measured. Experiments with this instru- 
ment show that there are two points, one 
in the northern hemisphere (latitude 70), 
the other in the southern (lat. 75), in 
which the needle stands vertical, and the 
dip is therefore 90 degrees. That, on the 
contrary, there is a circle of points near 
the equator, at which the needle is par- 
allel to the horizon, and the dip is ; 
this line is called the Magnetic Equator. 
At different intermediate points the dip is 
different, increasing, though not regular- ™e dipping needle. 

ly, as the distance from the magnetic equator increases. The dip, like the 
variation, keeps changing at a given place. At Washington it is now about 
70° 48', and is diminishing annually about 1' 48". 

879. The Compass. — The polarity of the magnetic 
needle, applied in the Compass, enables us to determine, at 
any place, a given direction or the bearing of a given object. 

The Land or Surveyor's Compass is simply a magnetic 
needle set in a shallow case covered with glass, on the 
bottom of which is a circular card, having its circumfer- 
ence divided into 360 degrees. At a distance of one-fourth 
of the circumference apart stand the letters N, E, S, W, 
denoting the four cardinal points — North, East, South, 
West. As the needle is stationary, while the card moves, 
the order of the points is reversed ; that is, when we hold 
the instrument so as to have the point S next to us, E is 
on the left, and W on the right. 




Lines of no Variation ? How many are there ? 878. What is the Dip of the Needle ? 
When and by whom was it discovered ? How may the Dip at any given place be 
measured? What is shown by experiments with the dipping needle? How great 
is the dip in the latitude of Washington ? 879. In what instrument is the polarity of 



342 



MAGNETISM. 



880. It is to the navigator, who relies entirely on it for 
guidance over the trackless ocean to his desired port, that 
the compass is most important. Arranged for his use, it is 
called the Mariner's Compass. 

Fig. 310. 1° t ne mariner's 

compass, repre- 
sented in Fig. 310, 
the circular card 
is attached to the 
needle and turns 
with it. The cir- 
cumference of the 
card is divided into 
32 equal parts, de- 
noted by marks and 
sometimes subdi- 
vided into halves 
and quarters. These 
marks have names 
given to them, in- 
dicating the dif- 
ferent directions, 
which are called 
Points of the Com- 
pass. Mentioning 
the points of the 

compass in their order is called boxing the compass. — The compass box is 
suspended within a larger box by means of two brass hoops, or gimbals as 
they are called, supported at opposite points on pivots, so that however the 
vessel may roll or pitch the needle may retain its horizontal position. 

It is believed that the Chinese were the first to avail themselves of the 
magnet in navigation, many hundred years before the Christian era; and 
that from them various other eastern nations learned to use it for the same 
purpose. The compass of these early times was probably nothing more than 
a piece of loadstone mounted on a cork and allowed to float on water. The 
magnetic needle and the card attached to it were no doubt the inventions of 
Europeans, among whom a knowledge of the rude compass used in the East 
appears to have been introduced in the twelfth century after Christ. Flavio 
Gioia [Jlah'-ve-ojo'-yah], a Neapolitan who flourished about the year 1300, 1 , 




THE MARINERS COMPASS. 



the magnetic needle applied ? Describe the Land Compass. 880. To whom is the 
compass most important? Describe the Mariner's Compass. What is meant by 
boxing the compass t How is the compass box suspended ? Who are thought to 
have been the first to use the magnet in navigation ? What did this ancient compass 
probably consist of? When did it first become known in Europe ? What improve- 
ments were soon made ? How did the name of Flavio Gioia become connected with 



THE COMPASS. 343 

by some regarded as the inventor of the compass, probably merely improved 
its construction, or extended its use among the maritime nations of Europe. 

No one can estimate how much the invention of the mariner's compass 
has contributed to the progress of the world. Relying on his little needle, 
which never betrays its trust, the mariner is no longer obliged to keep his 
bark within sight of land, and to direct his course by sun and star which 
clouds may obscure for days and nights together. He fearlessly ventures 
into unknown seas, explores the remotest regions, pursues his way under 
lowering skies and in utter darkness, well knowing whither he is sailing and 
how to steer when he wishes to retrace his course. This simple instrument 
has thus made the ocean a safe and frequented highway, extended the com- 
merce and knowledge of the world, linked its most distant families in friendly 
intercourse, and brought whole continents virtually into being. 

881. The compass needle, like all other magnetic nee- 
dles, is subject to variation and dip. 

Its variation seems to have been known two hundred years before the 
time of Columbus ; but that this variation differs in different places was dis- 
covered by that navigator on his memorable voyage across the Atlantic in 
1492. As he went westward, he observed that the variation increased from 
day to day. The fact was soon discovered by his crew, and filled them with 
consternation. It seemed ' as if the very laws of nature were changing, and 
they were entering a new world subject to mysterious influences'. It re- 
quired all the ingenuity of Columbus to induce them to proceed ; which he 
did by allaying their fears with an explanation of the phenomenon satisfac- 
tory to them, though it was far from satisfying himself. 

As the compass needle must be perfectly horizontal, the dip is counter- 
balanced by loading the end that tends to rise with a small weight, which 
may be shifted to suit any latitude. 

Tbeory of Magnetism. 

882. The theory of magnetism is analogous to that of 
electricity. Like electricity, magnetism is now regarded 
as a mode of force operating on ordinary matter, the mole- 
cules of which it polarizes, or arranges in a definite direc- 
tion. Every magnet is a collection of particles thus polar- 
ized, all the positive poles being turned in the same direc- 
tion, and the negative poles in the direction exactly oppo- 

the compass ? What is said of the effects which this simple instrument has wrought ? 
881. To what is the compass needle subject ? How long ago was the variation of the 
l^edle known ? What discovery did Columbus make respecting it ? What was the 
effect of this discovery on his crew? How is the dip counterbalanced in the com- 
pass needle? 882. To what is the theory of magnetism analogous? 1 How is mag- 



344 MAGNETISM. 

site. The positive or north pole of one particle is thus 
contiguous to the negative or south pole of the next. The 
opposite polarities are exhibited in full force at the extrem- 
ities of the magnetized body, but nullify each other at the 
centre. 

The molecules of loadstone are by nature constantly in 
this polarized condition. In some substances magnetic 
polarity is readily produced, and such are said to be easily 
magnetized ; the molecules of others are less susceptible 
to the polarizing influence, and others again can hardly 
be magnetized at all. 

When a piece of iron or steel is brought near the posi- 
tive pole of a magnet, its molecules at once become polar- 
ized by induction (§888). Their negative poles, attracted 
by the positive pole of the magnet, are directed towards 
the latter, at the extremity nearest to which the negative 
pole of the piece of iron or steel is thus established. Its 
positive pole is established at the opposite extremity, the 
positive poles of its molecules being repelled by the posi- 
tive pole of the magnet. In the case of soft iron, magnetic 
polarity ceases as soon as the polarizing agency is with- 
drawn, but in the case of steel it is permanent. 

883. Terrestrial Magnetism. — The polarity of the 
needle is best explained by supposing the earth itself to be 
a vast magnet. At the magnetic equator, as at the centre 
of a bar magnet, the opposite polarities neutralize each 
other, and there are no magnetic phenomena. Hence at 
this line there is no dip. The chief seats of magnetic 
energy are two points which lie towards the geographical 
poles of the earth, and which are called its Magnetic Poles. 

That point of the earth which attracts the north or positive pole of the 
needle, must be its south or negative magnetic pole. It lies near Hudson's 



netism now regarded ? How are the molecules of every magnet disposed? Where 
are the opposite polarities exhibited in full force? Why are they not exhibited at 
the centre of the magnetized body ? How do different substances differ, as regards 
the susceptibility of their molecules to polarizing influences ? What follows when a 
piece of steel is brought near the positive pole of suntignec? viS*>. Howisthe polarity of 
the needle explained ? Why is there no dip at the magnetic equator ? What is meant 



TEEKESTRIAL MAGNETISM. 345 

Bay, in 70 degrees of north latitude, and was reached by Captain Ross dur- 
ing his Arctic expedition of 1829. At this point he found the dipping needle 
to stand vertical, with its north pole towards the earth. The north or posi- 
tive magnetic pole of the earth has never been exactly reached, but is sup- 
posed to lie south of New Holland, in about 7-5° south latitude. The dipping 
needle would there also stand vertical, but with its south pole towards the 
earth. A point has been found near the region alluded to, in which the 
needle is very nearly vertical, the dip being 88 2 /3 degrees. 

The changes in the variation and dip appear to be in some way connected 
with the solar heat received by the earth. 

884. Magnets draw small pieces of iron to themselves ; but it must be 
remembered that the magnetic attraction of the earth only affects the direc- 
tion, and does not tend to change the actual position. A magnetic needle 
mounted on a cork and placed on the surface of a pond, is made to point north 
and south by the earth's magnetic attraction, but is not drawn to the north 
side of the pond. 

885. Magnetic Intensity. — A magnetic needle suspend- 
ed by a delicate fibre, when turned from the direction in 
which it naturally rests, resumes it, but not immediately. 
The magnetic attraction of the earth brings it back, but its 
inertia carries it past the point, and thus a series of vibra- 
tions, like those of a pendulum, take place before it finally 
settles. The number of such vibrations occurring in a 
given time evidently depends on the intensity of the earth's 
magnetic attraction. Now this number (and consequently 
the intensity of terrestrial magnetism) is found to be dif- 
ferent at different places, and at different times in the same 
place. 

The magnetic intensity varies according to the square of the number of vi- 
brations made in a given time. By applying this law, it is ascertained that 
the greatest magnetic intensity thus far found on the earth's surface is three 
times as great as the least. The magnetic intensity is found to be least in 
Southern Africa. 

Production of Artificial Magnets. 

886. Artificial magnets should be made of well hard- 
ened steel, of fine grain and uniform structure, free from 

by the Magnetic Poles ? Where is the earth's south magnetic pole ? By whom was 
It reached, and what was found there ? Where is the earth's north magnetic pole ? 
How near has it been reached ? With what do the changes in variation and dip seem 
to be connected ? 8S4. What alone is affected by the magnetic attraction of the 
earth? Give an illustration. 885. How is the intensity of the earth's magnetic at- 
traction shown to be different at different places ? What is the law for ascertaining 

15* 



346 MAGNETISM. 

flaws, and having level and polished faces. The breadth 
of a bar magnet should be one-twentieth of its length, and 
its thickness about one-seventieth of its length. In a horse- 
shoe magnet, the distance between the poles ought not to 
be greater than the breadth of one of the sides. 

887. Magnetism may be imparted to steel or iron in four 
different ways: — 1. By induction. 2. By the sun's rays. 
3. By contact with a magnet. 4. By electric currents. 

888. Induction, a source of Magnetism. — A magnetic 
atmosphere surrounds every magnet. A piece of iron or 
steel brought within this atmosphere, even without touch- 
ing the magnet, has its molecules endowed with polarity, 
and exhibits magnetic properties. It is then said to be 
magnetized by induction. 

Present half a dozen bars of iron at different angles to the positive pole 
of a magnet, without letting them touch it. They will all be magnetized by 
induction, the ends towards the magnet becoming negative poles and the 
opposite ends positive. 

Suspend two pieces of soft iron wire by threads, parallel to each other 
and on the same level. On bringing either pole of a magnet a short distance 
below them, they become magnetized by induction. Like poles are formed 
in their contiguous extremities, and consequently instead of hanging parallel 
as before, they repel each other and diverge. 

Bring one end of an unmagnetized steel bar near the north pole of a mag- 
netic needle, and the latter will be attracted to it. Now place the positive 
pole of a powerful magnet near the other end of the bar, and the needle will 
soon be repelled. This is because the bar becomes magnetized by induction. 
The end nearest the needle becomes a positive pole by which the positive 
pole of the latter is repelled. 

889. The earth magnetizes by induction. A bar of soft 
iron placed in the direction of the dipping needle, acquires 
magnetic properties by the inductive influence of the earth 
acting as a magnet. A few blows with a hammer on the 

the magnetic intensity ? What is found by applying this law ? Where is the mag- 
netic intensity found to be least? SS6. Of what should artificial magnets be made? 
What should be the comparative dimensions of a bar magnet? What is essential in 
a horse-shoe magnet? 8S7. Name the four ways in which magnetism maybe im- 
parted to a piece of steel or iron. 88S. When is a piece of iron said to be magnetized 
by indtictionf Illustrate magnetic induction with an experiment. Describe the 
experiment with two pieces of soft iron wire. What other experiment proves that a 
bar may be magnetized by induction ? 8S9. How is it proved that the earth mag- 
netizes by induction *! What experiment shows the inductive influence of the earth ? 



PRODUCTION OF ARTIFICIAL MAGNETS. 347 

upper end, by causing the particles to vibrate, help them 
to receive the magnetic influence. 

Hold a bar of soft iron horizontally with one end near the north pole of a 
magnetic needle. The iron, being unmagnetized, attracts the needle. Now 
hold the bar in the direction of the dipping needle, give it one or two blows 
with a hammer, and the north pole of the needle will be repelled, — showing 
that the bar is magnetized, and a north pole formed in its lower end, by the 
inductive influence of the earth. 

Iron bars that have long stood in a vertical position, or in the direction 
of the dipping needle, often acquire magnetic properties in an inferior de- 
gree. The same may be said of iron bars raised to a red heat and allowed 
to cool in the positions above mentioned, as well as of augers, gimlets, &c, 
that have been much used. Iron wire is frequently made magnetic by twist- 
ing it till it breaks. — All these are instances of magnetism by induction. 

890. The Sun's Rays, a source of Magnetism. — Sun- 
light constitutes a second source of magnetism. The violet 
rays of the solar spectrum, concentrated by lenses on steel 
needles, have been found to endow them with magnetic 
properties. 

891. Contact with a Magnet, a source of Magnet- 
ism. — A third and more efficient mode of exciting magnet- 
ism in iron or steel is by bringing it in contact with a mag- 
net. Till recently this was the way in which artificial 
magnets were almost exclusively produced. 

There are several different ways of magnetizing by con- 
tact. The principal are as follows : — 

892. Magnetizing Needles. — An ordinary sewing needle 
may be magnetized by simply touching one of its ends to 
either pole of a powerful magnet. The end in question be- 
comes negative if touched to the positive pole, and positive 
if touched to the negative. 

893. Magnetizing Bars. — Steel bars maybe magnetized 
either by single touch or double touch. Single Touch con- 
sists in applying but one pole of a magnet to the bar, or 
one pole to one-half, and the opposite pole to the other. 



Give some further instances of magnetism by the inductive influence of the earth. 
890. What is a second source of magnetism ? How may sun-light be made to mag- 
netize steel needles ? 891. What is a third source of magnetism ? 892. What is the 
mode of magnetizing needles ? 893. What two modes are there of magnetizing steel 




348 MAGNETISM. 

Double Touch consists in applying both poles at the same 
time throughout the whole length of the bar. 

894. To magnetize a bar by single touch, apply midway of its length one 
of the poles of a magnet, and draw it to either end. Return it through the 
air to the middle of the bar, and draw it again to the same end as before. 
Repeat this process several times, always using the same pole and drawing 
it in the same direction. Then place the other pole on the middle of the bar, 
and draw it to the opposite extremity, repeating the strokes as in the former 
case. This must be done on both sides of the bar. 

Fig. 311. s Another mode is repre- 

sented in Fig. 311. The op- 
posite poles of two magnets, 
kept about one-fourth of an 
inch apart by apiece of wood, 
are placed on the centre of 
the bar A B, so as to form angles of about 30 degrees with its surface. They 
ure then slowly drawn in contrary directions from the middle to the extrem- 
ities. This process is repeated several times, the magnets being raised when 
they reach the ends and replaced in the middle. The bar is then turned over, 
and the same thing done on the other side. The process is facilitated by 
resting the ends of the bar on the opposite poles of two other magnets, as 
shown in the figure. 

895. To magnetize a bar by double touch, apply the opposite poles of two 
magnets as just described, only let them be perpendicular to the surface. 
Then, instead of drawing them to opposite extremities as before, move them 
together from the middle to one end, then through the air to the opposite ex- 
tremity, and over the bar to the same end again, and so on — drawing them 
in the same direction over the bar, letting neither of the applied poles pass 
beyond its extremity, and finally stopping in the middle. 

Fig 312 . 896. Magnetizing Horseshoe Bars. 

— Horse-shoe magnets are produced by 
placing a piece of soft iron, as a keeper, 
across the ends of a steel bar bent in the 
proper form ; and then, as shown in Fig. 
312, applying perpendicularly to the ex- 
tremities a horse-shoe magnet, whose 
arms are the same distance apart. Move 
it slowly to the bend, then carry it back through the air to 
the extremities, and draw it to the bend again. This must 



bars ? In what does Single Touch consist ? In what, Double Touch ? 894. Describe 
the process of magnetizing a bar by single touch. What other mode is described ? 
s95i How is a bar magnetized by double touch? 896. How are horse-shoe magnets 





PRODUCTION OF ARTIFICIAL MAGNETS. 349 

be done about a dozen times ; then, without removing the 
keeper, turn the bar over and do the same on the other side. 
The poles of the magnet produced will in this case be of 
the same character as those respectively brought in con- 
tact with them. 

897. The best mode of magnetizing a 
horse-shoe bar is represented in Fig. 313. 
Lay the horse-shoe, A B, flat on a table, 
with its ends in contact with the poles of 
a horse-shoe magnet, N, S. Then place 
a piece of soft iron on these poles, and 
draw it slowly six or eight times towards 
the bend of the bar, in the direction of the arrow, raising it as often as it 
reaches the bend, and replacing it as at first. This process performed on 
both sides endows the horse-shoe with strong magnetic properties. The end 
which touches the positive pole of the horse-shoe magnet becomes negative, 
and the other positive. 

Two straight bars may be readily magnetized at once in the same way, 
by placing one extremity of each against the poles of the horse-shoe magnet, 
and connecting the opposite ends with a keeper. 

898. Electric Currents, a source of Magnetism. — 
A bar of iron or steel is endowed with magnetic properties 
in the highest degree, by passing a current of voltaic elec- 
tricity over a conductor placed in a certain position rela- 
tively to the bar. The details of this process belong to 
that branch of the science which is known as Electro- 
magnetism. 

Electro-magnetism. 

899. Electro-magnetism treats of the phenomena and 
principles of magnetism excited by the passage of electric 
currents. 

900. Effects of Electric Currents on the Magnetic 
Needle. — As a science, Electro-magnetism owes its origin 
to a discovery made in 1819 by Prof. Oersted, of Copen- 
hagen. He found that a wire along which a voltaic current 



produced? What will be the character of the poles in the magnet produced? 
897. With Fig. 313, describe the best mode of magnetizing a horse-shoe bar. How 
may two straight bars be magnetized at once ? S9S. How is a bar of steel endowed 
with magnetic properties in the highest degree ? 899. Of what does Electro-magnet- 
ism treat? 900. To what does electro-magnetism owe its origin? Give an account 



350 ELECTRO-MAGNETISM. 

was passing tended to turn the magnetic needle from its 
natural position to one perpendicular to the direction of 
the current. The conducting wire, of whatever metal it 
might be, was thus rendered magnetic by the electric cur- 
rent which it transmitted. It was subsequently found to 
attract iron filings ; which, when the battery was in full 
action, clustered around it to the thickness of a quill, but 
gradually thinned off as the energy of the battery dimin- 
ished, and left it entirely bare the moment the circuit was 
broken. 

The direction in which the needle is turned depends on its position rela- 
tively to the wire, and the direction in which the current is passing. When 
the needle is on a different level from the wire, that is, directly above or be- 
low it, it retains its horizontal position ; but its north pole is turned east or 
west, according to whether it is above or below the wire, and according to 
the direction in which the current moves. When the needle is on the same 
level with the wire, but on one side of it, it does not then swerve east or 
west ; but its north pole is made either to dip or to rise, according to the 
side of the wire it is on and the direction in which the current moves. The 
following rule enables us always to determine the direction in which the 
needle will be turned : — 

Imagine yourself, with arms extended perpendicularly, lying along the 
conducting wire, with your head towards the point from which the current is 
coming, and your face turned towards the north pole of the needle ; then this 
north pole will be deflected in the direction of your right hand, whether it b« 
up or down, east or west. 

The magnetic influence of the electric current is not therefore exerted in 
the plane of the conducting wire, but rather perpendicularly to that plane, 
so as to produce circular motion round the wire. 

901. The deflection of 'the needle by an electric cur- 
rent may be shown with the apparatus represented in 
Fig. 314. 

A brass wire is bent into rectangular form, and provided with a screw- 
cup at each extremity, P, N, for the reception of the wires from a galvanic 
battery, so that a current may be passed above and below a magnetic needle, 
N, S, suspended within the rectangle. The arms proceeding from P and N 



of Oersted's discovery. How was it proved that the conducting wire was rendered 
magnetic by the electric current ? On what does the direction in which the needle 
turns depend? How does it turn, when on a different level from the wire? How, 
when on the same level with the wire, but on one side of it ? State the rule for de- 
termining the direction in which the needle will be turned? How is the magnetic 
Influence of the electric current exerted ? 901. Illustrate the deflection of the needle 



THE GALVANOMETER. 



351 



!fc 





are insulated from each other Fig. 314 

where they cross. No sooner is a JP Jr 

positive current passed over the 

upper wire from north to south,. 

than the needle is turned, its A 

north pole deviating towards the 

east and its south pole to the 

west. 

Here the under current, pass- 
ing in the opposite direction to 
the upper one, tends to turn the 
needle in the same direction ; and the deflecting force, as it is called, is there- 
fore twice as great as if the current passed in one direction only. If the wire 
be bent so as to make two rectangles about the needle, the deflecting force 
will be twice as great as when but one is 
formed ; if five rectangles are made, as in 
Fig. 315, it will be five times as great, &c. 
In these cases, the wire must be covered 
with silk thread, or some other non-con- 
ductor, so as to insulate its arms from 
each other, and oblige the current to traverse its whole length. It is on this 
principle that the Galvanometer is constructed. 

902. TJie Galvanometer. — The Galvanometer is an in- 
strument for measuring the force of galvanic currents by the 
deflection of the magnetic needle. It consists of a long 
wire bent into an oval or rectangular coil, the parts of 
which are prevented from touching by being wound with 
silk. The wire terminates in screw-cups, for convenience 
of connection with a galvanic battery. Within the coil a 
magnetic needle is delicately poised ; and the instrument 
is placed so that the wire may have the same direction as 
the needle. They retain this direction till a galvanic cur- 
rent passes over the wire, when the needle is turned to- 
wards the east — more or less, according to the force of the 
current. A graduated scale fixed below the needle, with 
its circumference divided into degrees, measures the de- 
flection, and consequently the quantity of electricity passing 
over the wire. 



with Fig. 814 What is the effect of having two currents, one above and one below ? 
What is the effect of having two rectangles ? Five rectangles ? In these cases, what 
precaution must be taken? What instrument is constructed on this principle? 
i>02. What is the Galvanometer ? Describe the galvanometer. 903. How is the gal- 



352 



ELECTRO-MAGNETISM. 



Fig. 316. 




GALVANOMETER WITH ASTATIO 
NE1CDLE. 



903. Galvanometer with Astatit 
Needle. — Instead of the ordinary nee- 
dle, an astatic needle (see § 875) is 
sometimes used in the galvanometer. 
In this case, the needle, having its 
polarity neutralized, is more readily 
turned. The instrument is consequent- 
ly more sensitive, indicating the pres- 
ence of electric currents which would 
otherwise entirely escape detection. 

Fig. 316 represents the Galvanom- 
eter with the Astatic Needle. The nee- 
dles are suspended by two parallel silk 
threads from r, so that one of them 
may hang directly over the top of the 
coil z c, and the other below it. p q are 
the screw-cups terminating the wire 
which forms the coil, and ss is the 
graduated scale. The upper needle 
hangs above the coil ; but as its poles 
point in opposite directions to those of the under one, it will tend to move in 
the same direction as the latter when galvanic action takes place. 

904. Connection between Electricity and Magnet- 
ism. — That there is an intimate connection between elec- 
tricity and magnetism, was established by Oersted's experi- 
ment. It is further shown by the fact that compass-needles 
often have their poles reversed or their polarity weakened 
by lightning ; that a spark has been drawn from a magnet ; 
that a charge of electricity passed through a needle renders 
it magnetic ; and that a bar may be permanently magnet- 
ized with an electric current more efficiently than in any 
other way. 

These facts have led to the theory that magnetism is 
not an independent agent, but simply one of the forms as- 
sumed under certain circumstances by that polarizing force 
which, as most commonly exhibited in its action on the 
molecules of ordinary matter, we call electricity. Ac- 
cording to this theory, frictional electricity, voltaic elec- 
tricity, thermo-electricity, magneto-electricity, and electro 
magnetism, are but varied forms of one and the same thing, 



vanometer made more sensitive, and why? Describe the Galvanometer with tho 
Astatic Needle. 904. What was established hy Oersted's experiment ? How is the 
oonneotion between electricity and magnetism further shown? What theory has 



ELECTRO-MAGNETIC ROTATION. 353 

differing in intensity, quantity, and properties, in con- 
sequence of the different modes in which they are devel- 
oped. 

905. Electro-magnetic Rotation. — When a magnetic 
pole and a wire over which an electric current is passing 
are brought near each other, the pole tends to revolve 
round the wire, and the wire has a similar tendency to 
revolve round the magnet in a plane perpendicular to 
the direction of the current. With suitable apparatus, the 
following phenomena of electro-magnetic rotation may be 
exhibited : — 

1. The conducting wire being fixed, the magnet will 
revolve about it. 

2. The magnet being fixed, the conducting wire will 
revolve about it. 

3. Both magnet and wire being left free to move, 
they will revolve in the same direction round a com- 
mon centre, each appearing to pursue and be pursued by 
the other. 

4. The conducting wire being dispensed with, a magnet 
may be made to turn on its own axis by the passage of an 
electric current along half its length. 

906. To show the revolution of a magnet about a 
conducting wire, Faraday used the apparatus repre- 
sented in Fig. 317. A magnet, n S, is immersed in a 
vessel of mercury, with its north pole, n, a short dis- 
tance above the liquid, and its south pole, S, connect- 
ed by a silk thread with the conducting wire C, which 
passes through the bottom of the vessel, a b is an- 
other conducting wire, which enters the mercury from 
above. When a b is connected with the positive pole 
of a galvanic battery, and C d with the negative, a de- 
scending current of positive electricity passes along 
the conductor (the mercury completing the circuit), 
and the north pole, n, will revolve round the fixed 
wire, a b, in the direction of the hands of a watch. If, d * 
on the contrary, a b be connected with the negative 

been based on these facts ? 905 What follows when a magnetic pole and a wire over 
which an electric current is passing are brought near each other ? With suitable ap- 
paratus, what phenomena connected with electro-magnetic rotation may be exhibit- 
ed? 906. Describe Faraday's experiment for showing the revolution of a magnet 




854 



ELECTRO-MAGNETISM. 



pole, and C d with the positive, an ascending current will be formed, and the 
magnet will revolve in the opposite direction. 

Mercury is used in this experiment, because, being a liquid, it allows the 
magnet to move through it, while at the same time, being a conductor, it 



Fig. 318. 




completes the circuit, and carries off the magnetic in- 
fluence from the south pole immersed in it. Were it 
not for this, the south pole, by its tendency to move 
in the opposite direction to the north, would keep the 
magnet stationary. 

907. Fig. 318 illustrates the revolution of a con« 
ducting wire around a fixed magnet. Again we have 
a vessel of mercury, with a conducting wire, d, passing 
through its bottom, and another wire, a b, suspended 
from a hook directly over the magnet, entering the 
mercury from above, n is the north pole of the fixed 
magnet. On connecting the hook and the wire d with 
the poles of a galvanic battery, the wire will revolve 

sound the magnet, the direction depending, as before, on whether the electric 

current is ascending or descending. 

908. By ingeniously combining 
the two pieces of apparatus just de- 
scribed, we may exhibit the simulta- 
neous revolution of both magnet and 
wire round a common centre. The 
magnet, M, is immersed in a vessel 
of mercury about half its length, that 
the current may affect only one pole. 
It is connected at the bottom with a 
conducting wire and screw-cup, C, in 
such a way as to allow it freedom of 
revolution. The wire, W, is sus- 
pended from a hook, so as to move 
freely. On transmitting a current, 
which is done by connecting A and 
C with the poles of a battery, both 
the magnet and the wire commence 
revolving in the same direction as if 
chasing one another 

909. Effect of Electric 
Currents on Steel and Soft 
Iron. — The deflection of a 
magnetic needle by a wire 




about a conducting wire. Why is mercury used in this experiment ? 907. Describe 
the experiment which shows the revolution of a conducting wire around a fixed 
magnet. 908. "What does Fig. 319 represent ? Describe the experiment with this 



THE HELIX. 



355 




over which an electric current is passing, has been de- 
scribed in § 900. If a bar of soft iron is placed across such 
a wire, it becomes a temporary magnet, as is shown by its 
attracting iron filings. A bar of steel so placed is made a 
permanent magnet. 

910. The Helix. — The magnetizing power of the wire is 
greatly increased, if, instead of touching the bar in but a 
single point where they Fig. 320. 

cross, it is wound a number 
of times spirally round the 
latter, as shown in Fig. 320. 
Such a coil of wire is called a Helix (plural, hel'-i-ces). 

A helix may be familiarly made by winding some copper wire tightly 
round a small bottle, and then drawing the bottle out. As the magnetizing 
power of the helix increases with the number of times that the electric cur- 
rent passes round the bar, each turn of the wire is pushed close up to the 
one before it ; and, to increase the effect still further, several coils or layers 
of wire may be formed, one on top of another. Direct communication be- 
tween contiguous parts of the wire must be prevented by winding silk or 
some other insu- Fig. 321. 

lating material 
round it. When 
the ends of the 
wire are connect- 
ed with the poles 
of a galvanic bat- 
tery, the current 
is thus obliged to 
pass through its 
whole length. Fig. 
321 represents a A HBLi:s " 

helix mounted on a stand. An iron bar extending through the centre is seen 
projecting at each end. 

911. Magnetizing Power of the Helix. — A steel bar 
introduced within a helix becomes permanently magnetized 
the moment an electric current is passed over the wire. A 
needle laid inside of it is sometimes so powerfully acted on 




apparatus. 909. What is the effect of a wire over which a current is passing on a har 
of soft iron placed across it? On a bar of steel so placed? 910. How is the effect 
greatly increased ? What is such a coil of wire called ? How may a helix be made ? 
How is the effect of the helix increased ? With what is the wire covered, and why? 
What does Fig. 321 represent? 911. What is the effect of a helix on a steel bar in 



356 



ELECTRO-MAGNETISM. 



Fig. 322. 




as to be lifted up and held suspended in the air in the mid- 
dle of the helix. A bar of soft iron placed in the same po- 
sition is endowed with strong magnetic properties for the 
time, but instantly loses them when re- 
moved, or when the current ceases to 
pass. To be magnetized, the bar must 
always be placed lengthwise of the helix, 
— that is, at right angles to the direction 
in which the current is passing. 

One of the most remarkable effects of the helix is 
the suspension in the air, without any visible support, 
of a heavy iron bar loaded with weights. A helix 
consisting of a very long wire, forming several coils 
one upon another, and charged by a powerful battery, 
is held in a vertical position, as shown in Fig. 322. 
An iron bar brought within the helix just at its base, 
will be lifted up half way into it, and held there in 
the centre of the hollow cylinder, without touching 
it, as long as the current continues to pass. If pulled 
down a little way, it immediately springs back to its 
former position. The moment the current ceases, the 
bar falls. With a powerful apparatus, a weight of 
eighty pounds has been thus kept suspended in the 
air. 

A no less interesting experiment, 
showing the power of the helix, may be 
performed with the apparatus repre- 
sented in Fig. 323. The helix, A, is in 
the form of a ring. B, C, are two semi- 
circular pieces of soft iron, having their 
ends accurately fitted to each other. 
When B and C are brought together so 
as to form a circle, with one pair of their 
joined ends within the helix, they are 
endowed with so strong an attraction 
for each other that two men can hardly 
pull them apart. 

912. Electro-Magnets. — An 
electro-magnet consists of a bar of soft iron within a helix^ 




troduced within it ? On a needle ? On a bar of soft iron ? To be magnetized, how 
must the bar be placed? What is one of the most remarkable effects of the helix ? 
Describe the experiment Describe the experiment with the apparatus represented 



ELECTRO-MAGNETS. 



357 



It is strongly magnetic as long 
as a current passes over the wire, 
but loses its power the moment 
the current ceases. 

The most powerful electro-magnet is 
made by bending a bar of soft iron into the 
form of a horse-shoe, as shown in Fig. 324, 
and winding closely round it a large quan- 
tity of insulated copper wire so as to form 
a helix of several layers. The ends of the 
wire, Z, C, are connected with a powerful 
battery. A soft iron keeper, P N, connects 
the poles, having a hook beneath, to which 
weights may be attached. So strongly is 
this keeper attracted that an enormous 
force is required to separate it. An elec- 
tro-magnet prepared as above has sup- 
ported over 4,000 pounds. AN ELECT eo-magnet. 

913. Electro-magnets furnish us with the most efficient 
means of magnetizing an ordinary horse-shoe bar. The 
mode of using them for this purpose is shown in Fig. 325. 

Fig. 325. 





The electro-magnet is applied at the bend, one pole on each arm, and 
drawn towards the extremities, N, S. This is done several times on both 
sides, when the bar is rendered permanently magnetic. To deprive it of its 
magnetic power, reverse the process, by applying the poles of the electro^ 
magnet to the ends N, S, and drawing them towards the bend. 

914. Electro-magnetism, as a Motive Power. — We 
have seen that an electro-magnet is instantly endowed with 

in Fig. 323. 912. Of what does an electro-magnet consist ? How is the most power- 
ful electro-magnet made ? How great a weight has been supported with such an 
electro-magnet 9 913. What is the most efficient means of magnetizing a horse-shoe 



S58 ELECTRO-MAGNETISM. 

great attractive power for iron on being connected with a 
galvanic battery, and as instantly divested of it when the 
connection is severed. It may thus be made to impart 
motion to an iron rod, and through it to various kinds of 
machinery. So strong at one time was the impression that 
the enormous attractive power of the electro-magnet could 
be advantageously used as a mechanical agent, that the 
United States government appropriated $20,000, and Russia 
$120,000, for experiments on the subject ; and various ma- 
chines were contrived in which it was used as a motive 
power. In none, however, thus far invented, has it been 
found to approach steam in efficiency or economy. 

A boat 28 feet long with a dozen persons on board has been propelled 
against the current at the rate of three miles an hour by electro-magnetic 
action. A locomotive engine has also been driven from ten to twelve miles 
an hour. But this is the utmost that has been effected, and in both cases 
the cost of keeping the galvanic battery in operation was much greater than 
that of producing an equivalent quantity of steam. The difficulty appears to 
be twofold. First, the attractive power of the magnet rapidly diminishes as 
the distance from it increases. Secondly, electric currents opposite in direc- 
tion to the primary one are excited in the moving machinery ; which, in- 
creasing in power with its velocity, nullify much of the effect of the magnet. 
Until these difficulties are removed, electro-magnetism can not be advan- 
tageously used as a mechanical agent. 

915. The Electro-magnetic Telegraph. — Although 
unavailable as a motive power, electro-magnetism has been 
turned to practical account in the Telegraph, one of the 
crowning triumphs of human ingenuity. For this great 
invention as at present perfected, which enables us, almost 
with the rapidity of thought, to communicate with distant 
points, over miles of intervening land or sea, the world is 
chiefly indebted to an American — Samuel F. B. Morse. 

916. Morse's Telegraph. — The principles on which Morse's 
Telegraph operates are as follows : — 



bar ? Describe the process. 914 On what principle may an electro-magnet be mad* 
to impart motion to an iron rod ? For what were appropriations made by the United 
States government and Eussia? What has been effected with machinery moved by 
electro-magnetism? How does the expense compare with that of steam? What 
difficulties interfere with the usefulness of electro-magnetism as a motive power ? 
015. In what has electro-magnetism been turned to practical account? To whom is 



THE ELECTROMAGNETIC TELEGRAPH. 



359 



1. An electro-magnet may be alternately endowed with 
and deprived of the property of attracting iron by connect- 
ing and disconnecting it with a galvanic battery. 

2. The battery may be miles away from the magnet. If 
wires connect the two, the electric current will still be car- 
ried to the helix and produce the same effects. 

3. A person stationed near the battery may complete 
and break the circuit at pleasure. As he does so, one end 
of a lever placed near the poles of the distant magnet will 
be attracted or released. When it is attracted, the other 
end of the lever, which is furnished with a point, is made 
to indent a strip of paper passed in front of it by machinery, 
with dots or dashes, according to the time that the opera- 
tor by the battery keeps the circuit complete. If, now, 
different combinations of dots and dashes are agreed upon 
to represent certain letters, it is evident that a message can 
be communicated from the one point to the other. 

Fig. 326 represents Morse's recording apparatus. 




the world chiefly indebted for the Telegraph ? 916. State the principles on -which 
Morse's Telegraph operates. Describe Morse's recording apparatus, and its mode of 



360 



ELECTRO-MAGNETISM. 



A B is the electro-magnet, connected with the distant battery by the wires 
L, M, which are raised on poles and insulated by glass supports. C is an 
armature of soft iron attached to one end of the lever D D, so as to rest about 
one-eighth of an inch above the poles of the magnet. The other end of the 
lever carries a point or style, I, which is raised as C is depressed. A strip 
of paper, F, F, rolled on the spool E, is made to pass in front of the style, 
between the two cylinders Gr, H, by means of wheel-work set in motion by 
the weight J when the current passes. K is a spring, to pull down the end 
of the lever bearing the style when the other end is released by the magnet. 
A striking apparatus was formerly connected with the machinery in such % 
way as to give warning to the attendant with the first motion of the lever ; 
but it is now generally dispensed with, as the clicking sound produced by 
the lever is found to be sufficient for the purpose. 

Instead of carrying both wires over poles from the electro-magnet to the 
battery, the earth is now generally made to form one-half the circuit. This 
is effected by carrying down the wire from the magnet, and connecting it 
with a metallic plate buried in the ground ; a similar plate must be buried 
where the battery is stationed, and a wire from the latter connected with 
it. If this is done, but one wire need pass over the poles to complete the 
circuit. 

917. The apparatus used by the operator where the 
battery is stationed, to complete and break the circuit, is 
called the Signal Key. It is represented in Fig. 327. 

By pressing on the knob, 
Fig. 327. the screws in which the wires 

are fastened are connected, 
and the circuit is completed. 
On removing the hand, the 
knob springs up, the circuit is 
broken, and the current ceases. 
If the knob is kept pressed 
down, the paper at the other 
end is indented with a contin- 
uous line ; but by tapping on 
it so as to form different com- 
binations of dots and dashes, 
which stand for letters, and 
are understood at both ends of the line, a message is transmitted. Accord- 
ing to Morse's system, the following combinations are used to represent the 
different letters and figures : — 




THE SIGNAL KEY. 



operation. "What was formerly connected with the machinery ? Why is it now dis- 
pensed with? Instead of carrying hoth wires over poles from the electro-magnet to 
the battery, what is now the more usual arrangement ? How is the earth made to 
form half of the circuit ? 917. What is the Signal Key ? Describe it, and its mode 



morse's telegraph. 



361 





LETTERS 




FIGURES. 


a 


; 


s 


1 


b 


k 


t — 


2 


c - - - 


i 


u 


3 • 


d 


m 


v - - • — 


4 


e - 


n 


w 


5 


f 


o - 


3? 


6 


Or - - 


p 


y - • - - 


7 


h 


q -- 


z • • - 


8 


i - - 


r - 


$- - - - 


9 

■ 



To prevent confusion, a small space is left after each letter, a longer one 
between words, and a still longer one at the end of a sentence. The opera- 
tors in telegraph offices become so familiar with this alphabet that they un- 
derstand a message from the mere clicks of the lever, and the paper and 
wheelwork that moves it are now but little used. 

918. An electric current is transmitted by a wire to a 
great distance, but not with undiminished power. When, 
therefore, the stations are very far apart, the electro- 
magnet is charged too feebly to make the style indent the 
paper. In this case, the wire from the original battery is 
made to act on a very delicate armature, so as to complete 
the circuit of a second battery placed near the machine. 
This Relay Battery, as it is called, acts on the recording 
apparatus as described above, or transmits a fresh and vig- 
orous current to another relay battery. In this way lines 
of any length may be formed. 

As relay batteries do not interrupt the circuit, any number of them may 
be placed at intervals along a line. Each may work a recording apparatus 
of its own, and a given communication may thus be registered simultane- 
ously at a multitude of different stations. 

Eelay batteries may be dispensed with by increasing the number of 
plates employed and distributing them in groups along the line. It has 
been computed that, if a telegraph wire could be carried round the earth. 
1200 of Grove's pint cups, distributed in equi-distant groups of fifties, 
would supply the galvanic power for the whole distance. 



of operation. How are the different letters represented ? 918. What difficulty is 
there when the current is transmitted to a great distance ? How is this remedied ? 
How does the Eelay Battery act ? How may a given message be registered simulta- 
neously at different stations ? What may be substituted for relay batteries ? How 
many cups would supply the galvanic power for a telegraph round the earth? 
16 



362 ELECTRO-MAGNETISM. 

919. House's and Bain's Telegraph. — Morse's appara- 
tus, having been first introduced and being very simple, 
has been more used than any other, both in this country 
and in Europe. Other ingenious instruments have been 
employed to a greater or less extent. Among these are 
House's Printing Telegraph, Bain's Electro-chemical Tele- 
graph, Hughes's apparatus, and Phelps's instrument, an 
improved combination of the best features of those of 
House and Hughes. 

House's apparatus is one of the most wonderful achievements of invent- 
ive art. Making use of the electro-magnet in connection with ingenious and 
somewhat intricate machinery, it enables the operator, by playing on twenty- 
eight keys like those of a piano (representing the twenty-six letters and two 
punctuation points), to print ordinary letters on a strip of paper at the other 
end of the line at the rate of about two hundred a minute. The great advan- 
tages of House's system are that there is little or no liability to mistake in 
transmitting a message, and that the latter, being produced in Roman cap- 
itals, need not be transcribed, but may be sent just as it comes from the 
machine to the person for whom it is intended. 

In Bain's Electro-chemical Telegraph no magnet is used. The point of 
the wire, which is stationary, constitutes the pen, and rests lightly on a me- 
tallic plate, which is made to revolve by machinery. On this plate is plnccd 
paper which has been previously moistened with some chemical preparation 
decomposable by voltaic electricity. When the connection is made by the 
distant operator, the current passes from the wire to the plate through the 
paper, and in passing decomposes the chemical compound with which the 
paper is impregnated. The result is a deep blue spot on the paper, which 
renders the dot or dash visible, just as the indentation does according to 
Morse's system. As even a feeble voltaic current has the power of decom- 
position, there is not the same necessity for relay batteries on Bain's line as 
on either of the others. 

920. Submarine Telegraphs. — Submarine Telegraphs are 
telegraphs connecting points separated by water, in which 
the wire is submerged. The first successful telegraph of 
this kind was laid in 1851 across the English Channel, and 
connected Dover with the French coast. This was fol- 
lowed by several others ; and in 1858, after several unsuc- 
cessful attempts, a telegraph cable nearly 2,000 miles in 
length was laid across the Atlantic Ocean, between Valen- 

919. What other telegraph systems besides Morse's are in use? What is said of 
House's apparatus? What are its great advantages? What is the principle involved 
in Bain's Electro-chemical Telegraph? What advantage is there connected with this 
system? 920. What are Submarine Telegraphs? Where and when was the first 
«*nbmarine te!e<rranh laid ? In 1858 what ere..t p.nterorise was onrrieo' through 9 *V 



HISTORY OF THE TELEGRAPH. 363 

tia Bay, Ireland, and Trinity Bay on the coast of New- 
foundland. It consisted of a group of seven copper wires 
insulated and protected by a casing of gutta-percha, the 
whole surrounded by strands of iron wire, and sunk to the 
bottom of the ocean, at a depth nowhere exceeding 1\ miles. 
After transmitting several messages, this telegraph, owing 
to some fault in the cable, ceased to work, though obscure 
signals were from time to time received. In 1866, how- 
ever, another cable was successfully laid, and we now have 
regular telegraphic communication between the opposite 
sides of the Atlantic. 

921. History of the Telegraph. — The fact that frictional 
electricity could be conveyed by wires to a great distance 
was known more than a hundred years ago. Franklin, in 
1748, set fire to alcohol by means of a wire from an elec- 
trical machine carried across the Schuylkill River. The 
first attempt to transmit a communication by electricity, 
however, was made in 1774 by Le Sage \luh sahzh\ a 
Frenchman, at Geneva. 

Le Sage used twenty -four wires insulated in glass tubes buried in tbe 
eartb, eacb of which represented a letter of the French alphabet. The wires 
were connected with an electrical machine in the order necessary to spell out 
the words, and electroscopes attached to them at the other end indicated this 
order by their successive divergence to an attendant stationed there. 

922. Yolta's discovery in 1800 furnished afar more effi- 
cient agent for telegraphic communication than frictional 
electricity, and was followed in a few years by a plan for an 
electro-chemical telegraph, requiring thirty-five wires, to 
represent the different letters and figures, and to act by 
the decomposition of water. 

The great discovery of electro-magnetism in 1819 called 
forth many new suggestions, — among others, the use of the 
deflections of the needle as signals ; but none of the plans 
proposed were practicable on a large scale. A more per- 



scribe the Atlantic cable. How did it succeed ? When was the present cable laid f 
921. What fact relating to frictional electricity was known more than a hundred years 
ago ? What experiment was performed by Franklin in 174S ? Who made the first 
attempt to transmit a message by electricity ? Describe the plan of Le Sage. 922. By 
what was the discovery of voltaic electricity followed ? Whnr -ivr<"-tions were called 



364 ELECTRO-MAGNETISM. 

manent galvanic power was needed ; and this was not sup- 
plied till 1836, when Daniell brought out his constant bat- 
tery. The appearance of this battery and the improved 
electro-magnets prepared by Prof. Henry, was followed in 
1837 by the invention of apparatus for transmitting and 
recording communications, by Samuel F. B. Morse, who 
had been experimenting on the subject for five years. Ap- 
plication was at once made to the Congress of the United 
States for aid to construct a line of sufficient length to test 
the invention; and after discouraging delays, in 1843, the 
sum of $30,000 was appropriated by that body, with which 
a line was established between Baltimore and Washington, 
a distance of forty miles. The enterprise was crowned with 
complete success ; and the first news transmitted was the 
proceedings of the democratic convention of 1844, then 
sitting in Baltimore, by which James K. Polk was nomi- 
nated for the presidency. 

So manifold were the advantages of telegraphic communication, that im- 
mediately on the announcement of Morse's success companies were formed, 
and wires were soon seen threading the country in all directions. The va- 
rious lines now (1879) in operation in the United States and British Prov- 
inces make a total of about 110,000 miles, on four-fifths of which Morse's 
apparatus is used, that of Hughes and Phelps being chiefly employed on the 
remainder. With Morse's instruments about 9,000 letters may be trans- 
mitted in an hour. The cost of construction averages about $150 a mile. 

The same year in which Morse perfected his invention (1837), plans for 
telegraphic communication based on the deflections of the needle were an- 
nounced by Wheatstoue in England, and Steinheil [stlne'-hile], a German 
philosopher, to whom the discovery that the earth could be made to com- 
plete the circuit seems to be due. They are therefore sometimes mentioned 
as entitled to share with Morse the honor of his great invention. Their sys- 
tems, however, were but modifications of what had been proposed some years 
before; though practicable, they could not compete in rapidity of operation 
with Morse's, and consequently never came into general use. 

923. Electro-magnetic Clocks. — American ingenuity 

forth by the discovery of electro-magnetism ? By whom and when was the first per- 
fect apparatus for transmitting and recording communications invented ? What two 
improvements prepared the way for Morse's invention? How was Morse enabled to 
test his invention ? What was the result? What was the first news transmitted? 
How many miles of telegraph are now in operation ? On how much of this is Morse's 
apparatus used? What is the cost of constructing a telegraphic line? Who are 
sometimes mentioned as sharing with Morse the honor of inventing the telegraph? 



ELECTRO-MAGNETIC CLOCKS. 365 

has applied electro-magnetism to the determining of minute 
intervals of time and the regulation of clocks. The time 
of astronomical observations may thus be fixed with perfect 
precision to the tenth of a second. 

The pendulum of a clock, for instance, is, by some mechanical contrivance, 
made by its vibrations to close and break a galvanic circuit. With Morse's 
apparatus, each vibration is indicated by a dot on a strip of paper passed iD 
front of the style. If now an observer have a signal-key connected with the 
game circuit, by depressing it the instant a star passes one of the wires of 
his telescope, he permanently records its transit on the same paper by a dot 
intermediate between two vibration-dots, the exact time of which is known. 

924. By the same agency a number of clocks may be 
made to keep uniform time. 

This is effected by connecting any number of distant clocks, by means of 
wires, with one standard time-piece, which is itself connected with a gal- 
vanic battery, — so that the circuit may be closed and broken by all the pen- 
dulums simultaneously. Wheels connect the pendulums with the hands of 
the clocks, which are thus made to move with perfect uniformity. Some 
railroad companies use an arrangement of this kind to make the clocks at 
their different stations keep time together. 

925. Electro-magnetic Fire-alarm. — The principle of 
the telegraph has been used for raising a simultaneous alarm 
of fire at a number of different stations connected with one 
principal station by wires. By completing and breaking 
the galvanic circuit, an attendant who is constantly on watch 
at the principal station, and receives his information by tel- 
egraphic signals from the district in which the fire is de- 
tected, strikes alarm-bells at the various distant stations a 
certain number of times, according to the number of the 
district in question. Such an arrangement has been used 
in various cities with great success. 

926. The Helix, a magnet. — The helix, when traversed 
by a current of electricity, not only has high magnetizing 
powers, as we have seen, but is also itself a magnet. If 

What is said of their claims ? 923. To what has American ingenuity applied electro- 
magnetism ? Show how an astronomical observation may be telegraphically record- 
ed. 924. How may a number of clocks be made to keep uniform time by means of 
electro-magnetism ? 925. For what has the principle of the telegraph been used ? 
Show how an alarm of fire may be simultaneously raised at different stations. 
-926. What is the effect of an electric current traversing a helix on the helix itself? 



366 MAGNETO-ELECTRICITY. 

suspended so as to allow it freedom of motion, it points 
north and south, and dips like the magnetic needle. So, 
like poles of two helices repel each other ; unlike poles at- 
tract each other. 

Even when not bent in the form of helices, two wires traversed by elec- 
tric currents, if brought near each other in parallel lines and free to move, 
exhibit mutual attraction or repulsion. When their currents move in the 
same direction, they attract each other ; when in contrary directions, they 
repel each other. 

Magneto-electricity. 

927. Not only is magnetism developed by electric cur- 
rents, but electric currents are produced by magnetism. 
That branch of science which treats of electric currents so 
produced is called Magneto-electricity. — It is through 
magneto-electric currents that the recently-invented Tel- 
ephone reproduces sound-waves at distant points. See 
page 453. 

928. Experiments. — Connect the ends of wire from a helix with a galva- 
nometer. Then quickly thrust into the helix one of the poles of a bar 
magnet. The needle of the galvanometer is at once deflected, showing 
the passage of an electric current over the wire. If the opposite pole is 
introduced into the helix, a current passes in the contrary direction. 

Within a helix place a soft iron bar of such length that each end may 
project a little. Over its ends bring the poles of a horse-shoe magnet, so 
suspended as to have freedom of revolution. On turning the magnet rapid- 
ly, the poles of the bar are reversed twice for each revolution, and an elec- 
tric current is produced on the wire, as is shown by a galvanometer attached 
to it. This principle has been applied in different magneto-electric ma- 
chines, with which water may be decomposed, platinum wire heated to 
redness, sparks produced, shocks given, and other experiments performed. 

929. The Magneto-electric Machine. — Fig. 328 
represents one form of the Magneto-electric Machine. 

S is a compound horse-shoe magnet supported on three pillars. In front 
of its poles, and as near as it can be brought without touching, is a bar of 
soft iron bent at right angles, and surrounded with several coils of insulated 
copper wire. The ends of this wire are pressed by springs against a con- 



Prove that it renders the helix magnetic. What phenomena are exhibited by two 
straight wires traversed by electric currents, when brought near each other? 927. 
What is Magneto-electricity? In what instrument is it turned to account? 928. 
What is the first experiment illustrative of magneto-electricity? The second? 
How is the principle here described applied? 929. Describe the Magneto electrio 



MAGNETO-ELECTRIC MACHINE. 

Fig. 328. 



367 




MAGNETO-ELECTRIO MACHINE. 

ducting metallic plate, connected by wires passing under the stand with the 
screw-cups A, B. The soft iron armature just described is mounted on an 
axis which is made to revolve by a wheel turned by a handle. The handle 
being rapidly turned, each half-revolution of the armature brings its extrem- 
ities near opposite poles of the magnet, thus reversing its polarity, and pro- 
ducing a strong electric current on the wire. If small copper cylinders 
attached to the wires are grasped one in each hand, as shown in the figure, 
a series of severe shocks are received, and the muscles are so contracted that 
it is almost impossible to open the hands and let go the conductors. 

Machines of this kind, adapted to medical use, have been found effica- 
cious in cases of rheumatism, dyspepsia, sprains, nervous diseases, &c, the 
current being made to pass through the diseased part. 

IHamagnetism. 

930. Experiments with powerful electro-magnets show 
that almost all substances are susceptible of magnetic in- 
fluence. Some are attracted by the magnet ; others, re- 
pelled ; while a few are not acted on at all, though when 
more powerful magnets shall be made they will probably 
be found to fall under one of the two previous classes. . 

Hence arises a three-fold division of bodies. 1. Mag- 
netic bodies, or such as are attracted by an electro-magnet. 



Machine represented in Fig. 328, and its mode of operation. What is the effect of 
such a machine on the human system ? What use has been made of machines of this 
kind? 930. What has been shown by experiments with powerful electro-magnets? 
Name the three classes into which bodies are divided with reference to the influence 



368 



DIAMAGNETISM. 



2. Diamagnetic, or such as are repelled, 
such as are not acted on at all. 

Fig. 329. 



3. Indifferent, or 



The difference between these three classes 
of bodies may be illustrated with the apparatus 
shown in Fig. 329. N, S, are the poles of an 
electro-magnet, which is connected by the wires 
C, Z, with a galvanic battery. A bar of iron, 
nickel, cobalt, manganese, or other magnetic 
substance, suspended between the poles so as to 
move freely, will come to rest with its ends as 
near them as possible, in the position II. On 
the contrary, a bar of bismuth, phosphorus, zinc, 
tin, or other diamagnetic substance, similarly 
suspended, will be repelled and come to rest at right angles to the position 
just described, as shown by the dotted line, — with its sides opposite the poles 
of the axis and its ends as far from them as possible. Similar attraction and 
repulsion are exhibited if the substances are presented to either pole sepa- 
rately. An indifferent substance will remain in any position in which it is 
placed, being neither attracted like the iron nor repelled like the bismuth. 

Similar experiments may be made on liquids and gases by enclosing them 
in tubes. It is thus found that oxygen is magnetic ; water, alcohol, ether, 
and the oils, diamagnetic. 




CHAPTER XVIII. 

ASTRONOMY. 

931. Astronomy is the science that treats of the heav- 
enly bodies, — their motions, size, distance, &c. 

By the heavenly bodies are meant the sun, the moon, 
stars, planets, and comets. 

932. Astronomy, as it is the most sublime, is also the oldest of sciences. 
The shepherds of the patriarchal age, tending their flocks by day and night 
beneath the canopy of heaven, naturally directed their gaze to the brilliant 



sxerted on them by electro-magnets. Define each. Illustrate the difference be- 
tween these three classes with the apparatus represented in Fis;. 329. How may sim- 
ilar experiments be made on liquids and gases? What gas is found to be magnetic? 
What liquids are diamagnetic? 

931. What is Astronomy ? What are meant by the heavenly bodies ? 932. Who 



ASTKOXOMY. 369 

orbs with which it is studded, observed their motions, and thus became the 
first astronomers. Chaldean observations are said to extend back to within 
a hundred years of the flood. The Chinese, also, paid great attention to this 
science in remote antiquity. We are told that more than 2,000 years before 
the birth of Christ, an emperor of China put to death his two chief astrono- 
mers for not predicting an eclipse of the sun. 

Destitute of the admirable instruments which modern science has pro- 
duced and used with signal success, the ancient astronomers of course erred 
in many of their conclusions. We can only wonder that they obtained as 
much knowledge as they did respecting the heavenly bodies. 

933. To unfold the principles of astronomy at length 
would require a volume, and to understand them thorough- 
ly, a knowledge of the higher mathematics is essential. We 
can here present only such leading facts as will serve to 
give a general view of the science. 

934. Fuxdasiextal Facts. — The great facts established 
by the researches of astronomers are as follows : — 

1. Space is filled with worlds. 

Looking up into the heavens on a clear night, we see them all around us. 
The telescope reveals millions. There are no doubt millions more too remote 
to be seen at all, and others which from being non-luminous escape our vis- 
ion. Powerful instruments reach to points from which light, travelling as it 
does with the enormous-velocity of 185,000 miles in a second, would be 60,000 
years in reaching us, and throughout the whole of this vast field worlds are 
everywhere scattered. We can but infer that the regions to which man's eye 
has never penetrated are similarly studded ; and that, if an observer could 
be transported to the remotest star visible with his telescope, he would see 
spread before him in the same direction a firmament no less rich and splendid 
than that which he beheld from the earth. 

2. These worlds are divided into systems, the members 
of which are bound together by mutual attraction. Each 
system has a central sun, round which the other members, 
called Planets, revolve. While this revolution is going on, 
the suns themselves with their respective planets move 
about a common fixed central point. 

3. The stars that we see twinkling in the sky are suns. 

were the first astronomers ? How far back are Chaldean observations said to extend? 
What story shows the attention paid to astronomy by the Chinese in remote anti- 
quity? What is said of the ancient astronomers ? What js the first great fact estab- 
lished by astronomers? What facts are stated respecting the number of worlds? 
What inference is drawn respecting the regions of space impenetrated by the eye of 
man? How are these worlds divided? What are the stars that we see twinkling 



370 ASTRONOMY. 

The planets that we suppose to revolve about them are 
non-luminous, and therefore invisible. 

4. Some of these planets have satellites or moons moving 
around them, and with them around the sun of the system 
to which they belong. 

5. The Earth, which we inhabit, is a planet belonging 
to what is known as the Solar System, of which the Sun is 
the centre. The Earth is attended by one satellite known 
as the Moon. 

The Solar System. 

935. The Solar System, as at present known (1879), 
consists of the Sun, its centre ; two hundred and two. 
planets revolving round it, of which one hundred and 
ninety-four, on account of their small size, are called As- 
teroids {star-like bodies) ; twenty moons revolving round 
the planets ; and many thousands of comets, the exact 
number of which is unknown. 

936. That the earth and other planets move round the sun, was taught by 
the philosopher Pythagoras about 500 b. c. Deceived by appearances, how- 
ever, the ancients generally rejected this theory, and believed the earth to be 
the fixed centre of motion for all the heavenly bodies. Some made the plan- 
ets revolve round the sun, and the sun carrying the planets with it to move 
round the earth. The Egyptian astronomer Ptolemy supposed the universe 
to consist of a number of hollow spheres arranged one within another, and 
appropriated respectively to the sun, the moon, the planets, and the stars. 
The earth, according to Ptolemy, was at the centre of these spheres, which 
turned round it from east to west every twenty-four hours, carrying the stars 
and planets with them ; being of crystal, they were perfectly transparent, 
and the inner ones did not therefore obscure the more distant luminaries 
seen through them. 

These theories, particularly Ptolemy's, prevailed till about the middle of 
the sixteenth century, when the Prussian philosopher Copernicus revived the 
teachings of Pythagoras, and established what is called from him the Coper- 
nican System, which is now acknowledged as the true theory of the universe. 
Fearing the prejudices of his fellow-men, Copernicus withheld his system 
from them for some years. His great work, in which his views were embod- 



in the sky? Why are not their planets visible? By what are some of the planets 
attended? What is the Earth? By what is it attended? 935. Of what does the 
Solar System, as at present known, consist? 936. What was Pythagoras's theory of 
the universe ? What was the belief of the ancients generally? Give an account of 
Ptolemy's theory. By whom and when was the true system revived ? When wae 



THE SOLAR SYSTEM. 37 1 

ied, was finally published in 1543, just in time for a copy to be placed in his 
hands on his death-bed. 

The Copernican system at first met with but moderate favor. Its truth, 
however, was established by Galileo, whose observations with the newly- 
invented telescope afforded him incontrovertible arguments in its favor. Yet 
the advocates of the old system were determined to close their eyes. On 
Galileo's announcing the discovery of four moons about the planet Jupiter, 
they denied the possibility of their existence ; and when he urged them to 
look for themselves through his telescope, they refused to have anything to 
do with an instrument they despised. An astronomer of Florence gravely 
argued that as there were only seven apertures in the head — two eyes, two 
ears, two nostrils, and one mouth — and as there were only seven metals, and 
seven days in the week, so there could be only seven planets. As there were 
six principal planets and one moon then known, the number was complete, 
and Galileo's pretended planets must be impossibilities. — But such absurd 
arguments could not long obscure the light of truth. 

937. The Sun (#).— The Sun, the great source of light 
and heat to the planets, is the centre of the solar system. It 
is an immense globe, seven hundred times as large as all its 
planets put together. Its diameter is 852,000 miles. Placed 
where the earth is, it would fill the whole orbit of the moon, 
and extend 180,000 miles beyond it in all directions. Its 
volume is nearly a million and a quarter times as great as 
the earth's, and it contains more than 300,000 times as 
much matter. 

938. Solar Spots. — Viewed through a telescope, the sun 
looks like a globe of lire. Its surface, however, is not al- 
ways wholly luminous. A number of dark spots, surround- 
ed by a lighter shadow, are at times scattered here and 
there within a zone extending 35 degrees on each side of 
the solar equator. The number and size of these spots 
differ at different times ; for, while some last a couple of 
months or even longer, others change their form from day 
to day. They have been known to vanish almost instantly 
and to appear as suddenly. Some years none at all are 
visible ; in others, as many as 200 are seen at once, cover- 

the work of Copernicus relating to this subject published ? By whom was the truth of 
the Copernican system established ? What were the arguments with which Galileo 
was met ? 937. What is the Sun ? How great is it s diameter ? ' Placed where the earth 
is, how far would it extend ? How does its volume compare with the earth's? Its 
matter? 938. How does the sun look, when viewed through a telescope? Describe 
the spots which are sonK'tknes visibk-. What is said of their number and size ? What 



372 ASTRONOMY. 

ing so much of the surface as materially to diminish the 
quantity of light emitted. 

By comparing a number of observations on the solar spots, we find that 
they are subject to periodical increase and decrease. They become larger 
and more numerous for a certain time till they reach a maximum, after which 
they gradually diminish, till all disappear, or nearly so ; new ones then be- 
come visible, and go on increasing during the same period as before. This 
period seems to be a little over eleven years. 

Spots have occasionally appeared of such size that they could be readily 
discerned with the naked eye. One thus seen for a week in June, 1843, must 
have been 77,000 miles across, or nearly ten times the size of the earth. 

Astronomers have tried to account for the solar spots in various ways. 
The prevailing opinion is that the light received from the sun comes from 
a luminous surface, called the photosphere, consisting of various incandescent 
substances, the vapor of which forms an atmosphere of great height sur- 
rounding the photosphere. It is thought that the spots may be due to the 
absorption of the solar light by a greater thickness of atmosphere in certain 
places ; or, what is more likely, that they are portions of a less bright 
surface beneath, exposed to view when the luminous surface is opened by 
periodical upward currents or some other natural agency. The disturbance 
of the luminous surface, by whatever it is caused, is most frequent near 
the solar equator. — Peculiarly bright streaks of light, called faculce, are 
often found near the spots or where they have just disappeared. They 
are supposed to be the ridges of vast waves in the luminous atmosphere. 

939. Constitution of the Sun. — The sun's density is 
about one-fourth that of the earth. Respecting its interior 
constitution nothing is known; but examinations of the 
solar spectrum with an instrument called the spectroscope, 
show that the vapors of several metals known on the earth 
are present in its atmosphere, and. that these metals are 
therefore in its photosphere in a state of incandescence. 
Among the substances thus detected are sodium, iron, 
magnesium, copper, zinc, calcium, and nickel. 

940. Motions. — The more permanent of the sun's spots, 
if observed from time to time, are found to change their 
position on its disk, or face. First becoming visible on the 

is found by comparing a number of observations on the solar spots? What is the 
length of the period? Of what size have spots occasionally appeared? What was 
the diameter of one seen in dune, 1843? What is the opinion of astronomers re- 
specting these spots ? What are faculce? What are they supposed to be ? 989. How 
does the sun's density compare with the earth's? What has been discovered re- 
specting its atmosphere and photosphere ? 940. How is it proved that the sun 



THE ZODIACAL LIGHT. 373 

east side, they gradually move towards the west, and in 
about thirteen days are lost from sight in that direction. 
After a similar period they reappear in the east. This 
phenomenon shows that the sun turns on its axis from 
west to east ; its revolution is performed in about 25 days, 
8 hours. 

Besides turning on its axis, the sun, attended by its 
planets, moves at the rate of 8 miles a second in a circular 
path round a centre far off in the fields of space. So vast 
is this path that it will take the sun 18,200,000 years to get 
once completely round it. 

941. The Zodiacal Light. — A faint light, shaped like a 
sugar-loaf, is sometimes seen stretching obliquely upward 
in the heavens, from 70. to 100 degrees, from that part of 
the horizon where the sun is about rising or has just set. 
This phenomenon is known as the Zo-di'-a-cal Light. It is 
brightest and most distinctly defined in tropical regions, 
where it is visible most of the time. In high latitudes it is 
seldom clearly seen, except during March and April just 
after sun-set, and in September and October immediately 
before dawn. 

Some have supposed the zodiacal light to be an expansion of the solar 
atmosphere ; others, a thin vapor, charged with matter from the tails of 
comets, of which the sun's attraction has deprived them ; others, again, 
that it is a remnant of the original matter of which both sun and planets 
were formed. The latest theory is, that it is produced by countless mete- 
ors, revolving round the sun, and combining to reflect its light to our eyes. 

The Planets. 

942. By the Planets of the solar system are meant those 
heavenly bodies that revolve directly about the sun in ob- 
long curves, and shine by its reflected light. 

The word planetes in Greek means " a wanderer", and the bodies in ques- 
tion are so called in contradistinction to the fixed stars, which keep the same 



turns on its axis? What is the time of its revolution ? What other motion has the 
sun ? How large is the path it travels ? 941. Describe the Zodiacal Light. Where 
is it brightest ? When is it seen in high latitudes? What opinions have been ad- 
vanced to account for the zodiacal light ? 942. What are the Planets ? What does 
the word planetes mean? From what are the planets to be distinguished? How 



374 ASTRONOMY. 

position in the heavens relatively to each other. The planets and the fixed 
stars are easily distinguished ; the former shine with a steady light, the latter 
twinkle. 

943. The moons are sometimes called Secondary Plan- 
ets. In that case, the bodies that revolve directly about 
the sun are called Primary Planets. 

944. The planets are also distinguished as Inferior and 
Superior. The Inferior Planets are those that are nearer 
to the sun than the earth is ; the Superior Planets are those 
that are farther from the sun than the earth is. 

945. Orbits of the Planets. — The path of a planet 
round the sun is called its Orbit. The planets being at 
different distances from the sun, their orbits differ in length, 
though they are similar in shape. 

946. The planetary orbits are* not circles, but oblong 
curves called Ellipses. Hence a planet is nearer the sun in 
one part of its course than in another. That point of its 
orbit at which it is nearest the sun is called its perihelion 
(plural, perihelia) ; that in which it is farthest from the sun 
is its aphelion (plural, aphelia). When a planet's distance 

from the sun is spoken of, its mean dis- 
tance is meant. This is obtained by add- 
ing its greatest and least distance to- 
gether and dividing by 2. 

These definitions are illustrated in Fig. 330. 
A.BP C represents an ellipse. S is the sun, situ- 
ated not at the centre of the ellipse, but at one of two 
points within it called foci. P shows the position 
of a planet at its perihelion, and A at its aphelion. 

The orbits of the planets lie in different planes, 
more or less inclined to each other. 

947. Besides their revolution round the sun, the planets 
have another motion round their own axes. The time that 



can the planets and the fixed stars be told apart? 943. What constitutes the differ- 
ence between Primary and Secondary Planets ? 944. Between Inferior and Superior 
Planets? 945. What is a planet's orbit? 946. What is the shape of the planetary 
orbits ? What is a planet's Perihelion ? Aphelion ? When a planet's distance from 
the sun is spoken of, what is meant ? How is the mean distance obtained ? Illus- 
trate these definitions with Fig. 330. What is said of the planes of the orbits? 
94T. What other motion have the planets besides their revolution round the sun ? 




THE PLANETS. 



375 



it takes a planet to make one revolution on its axis is called 
its Day. 

948. Table. — A Table of the planets follows, in the or- 
der of their distances from the sun, which are given in the 
second column. Their diameters in miles are given in the 
third column ; the number of our days that it takes them 
to revolve round the sun, in the fourth ; and the hours re- 
quired for the revolution of each on its axis, in the fifth. 
The Tables in the American Edition of Lockyer's " Ele- 
ments of Astronomy" (1879) are here followed. 



Name. 


Distance from 


Equat'l diam- 


Year expressed in 


Sidereal day in 


Sun in miles. 


eter in miles. 


the Earth's days. 


hours, &c. 


Mercury . 


35,393,000 


2,962 


88 


24 h 5 m 28 s 


Venus . . 


66,131,000 


7,510 


225 


23 16 19 


Earth . . 


91,430,000 


7,926 


365y 4 


23 56 4 


Mars . . 


139,312,000 


4,920 


687 


24 37 23 


Asteroids 


j from 201 to 
( 313 millions 


j from 17 
( to 228 


j from 1,193 
( to 2,385 


unknown 


(194) 


Jupiter 


475,693,000 


85,390 


4,333 


9 55 28 


Saturn . . 


872,135,000 


71,904 


10,759 


10 29 17 


Uranus 


1,753,851,000 


33,024 


30,687 


unknown 


Neptune . 


2,746,271,000 


36,620 


60,127 


unknown 



949. Mercury, Venus, Mars, Jupiter, and Saturn, being visible to the 
naked eye, were known to the ancients. Uranus was discovered in 1781 by 
Sir William Herschel, from whom it was first commonly called Herschel. Its 
discoverer gave it the name of Georgium Sidus, in honor of King George III. 
Both these names, however, were discarded for the mythological one by 
which it is at present known. The first of the asteroids, Ceres, was discov- 
ered in 1801 by the Sicilian astronomer Piazzi [pe-at'-ze]. Pallas was added 
to the list in 1802; Juno, in 1804; Vesta, in 1807; and the remainder, since 
1844. 

Neptune was discovered in 1846 by Dr. Galle [gal'-la], of Berlin. It was 
first called Le Verrier [luh va-re-a'], in honor of an eminent French astrono- 



What is meant by a planet's day ? 948. Keferring to the Table, which of the planets 
do you find the smallest (the asteroids excepted), and Avhich the largest? "Which 
takes the shortest time to revolve around the sun, and which the longest? Which 
three have a day very nearly as long as the Earth's ? Which two have days less than 
half as long as the Earth's? 949. Which of the planets were known to the ancients? 
Which was the next discovered ? What other names has Uranus borne ? When and 
by whom was the first asteroid discovered? When were the rest added to the list? 
When and by whom was Neptune discovered? What was it first called, and whyT 



876 



ASTRONOMY. 



mer, who by a series of calculations established the fact that there was a 
more distant planet than Uranus, and instructed Dr. Galle in what part of the 
heavens to look for it. 

950. Bode's Law. — By comparing the distances of the 
planets from the sun, Bode [bo'-da] arrived at the following 
law : — Take the geometrical progression 

3 6 12 24 48 96 192 384, 
each term of which (after the second) is obtained by doub- 
ling the preceding one. To each term add 4, and we get 

4 V 10 16 28 52 100 196 388. 
The distances of the nearer planets are approximately pro- 
portioned to these numbers. That is, Mercury's distance be- 
ing 36,890,000 miles, Venus's will be f- as much, the Earth's 
V ; &c. Bode's law, however, does not apply to Saturn, 
Uranus, and Neptune. They are all, particularly the last, 
much nearer the sun than this law would make them. 

951. Fig. 331 shows the comparative size of the planets. 
The asteroids are too small to appear on this scale. 

Fig. 331. 
5 °° • % o g § 




Herschel uses the following illustration to give an idea of the relative size 
of the planets and their orbits : — " Choose any well levelled field or bowling- 
green. On it place a globe two feet in diameter ; this will represent the Sun 
Mercury will be represented by a grain of mustard-seed, on the circumference 
of a circle 1(54 feet in diameter for its orbit ; Venus a pea, on a circle of 284 
feet in diameter; the Earth also a pea, on a circle of 430 feet; Mars a rather 
large pin's head, on a circle of 654 feet ; the Asteroids, grains of sand, in 
orbits of from 1000 to 1200 feet; Jupiter, a moderate-sized orange, in a circle 

950. State Bode's Law. 951. What does Fig. 331 show ? Eepeat the illustration used 



KEPLER'S LAWS. 



377 



nearly half a mile across ; Saturn a small orange, on a circle of four-fifths 
of a mile ; Uranus a full-sized cherry, or small plum, upon the circumference 
of a circle more than a mile and a half; and Neptune a good-sized plum, 
on a circle about two miles and a half in diameter." 

952. Kepler's Laws. — The laws that regulate the plan- 
etary motions were unknown till the commencement of the 
seventeenth century, when, after a long and careful com- 
parison of numerous observations, they were discovered by 
John Kepler, a celebrated German astronomer, who thus 
won the title of " the Legislator of the Heavens". Kep- 
ler's laws apply to the moons in their revolutions about 
their primary planets, as well as to the latter. 

953. Kepler's First Law. — The orbits of the planets are 
ellipses having one focus in common, and in this common 
focus the sun is situated. 

The principal forces acting on the planets are the sun's attraction and the 
original force of projection. These forces alone would cause each planet to 
move about the sun in a perfect ellipse. The attraction of the other heavenly 
bodies, however, produces Perturbations, as they are called, and thus each 
orbit constantly deviates in a slight degree from an ellipse. 

The ellipses described by the planets differ from circles in different de- 
grees. The orbits of Mercury and several of the asteroids deviate most ; 
those of Neptune and Venus are nearly circular. 

954. Kepler's Second Laio. — The 
Radius Vector of a planet passes over 
equal areas in equal times. 

The Radius Vector is a line con- 
necting the centre of a planet, as it 
traverses its orbit, with the centre 
of the sun. 

Thus, in Fig. 332, the lines S A, SB, S C, 
<fec, represent the radius vector of the planet 
there traversing its elliptical orbit. The whole 
space included within the orbit is divided into 
12 equal triangles, 1, 2, 3, 4, &c. ; and these, 




by Herschel to give an idea of the relative size of the planets. 952. When were the 
laws that regulate the planetary motions first known ? By whom were they discov- 
ered ? To what do Kepler's Laws apply ? 953. Repeat Kepler's First Law. How is 
the elliptical form of the orbits accounted for ? What is said of the ellipses described 
by the planets ? Which of the orbits deviate most from a circle ? Which deviate 
very little ? 954. What is Kepler's Second Law ? What is the Radius Vector J Illus- 



378 ASTRONOMY. 

according to the law just stated, must be traversed by the radius vector in 
equal times. 

It follows from this law that the velocity of a planet differs at different 
points of its orbit, being greatest at its perihelion, and least at its aphelion. 
A B, CD, and the other arcs that form the bases of tbe twelve triangles, 
differ in length, but have to be traversed in the same time. The planet must 
therefore move fastest over the longest arcs, which are at its perihelion, and 
slowest over the shortest arcs, which are at its aphelion. In going from its 
aphelion to its perihelion, the arcs keep increasing, and the velocity of the 
planet is accelerated ; from its perihelion to its aphelion, the arcs keep di- 
minishing, and the velocity of the planet is retarded. Yet in going the whole 
distance from its aphelion to its perihelion a planet takes precisely the sam« 
time as in performing the opposite half of its course. 

The cause of this difference of velocity is easily explained. In travelling 
towards its perihelion, a planet is constantly acted on by the sun's attraction 
in the same general direction as that in which it is moving, and this attrac- 
tion becomes stronger and stronger as it approaches the sun. When return* 
ing to its aphelion, on the contrary, it is acted on by the sun's attraction in 
a direction opposite to that in which it is moving. 

955. Kepler's Third Lato. — The squares of the planets'* 
times of revolution round the sun are proportioned to the 
cubes of their distances from the latter. 

For example, Mercury's year consists of 88 days, Venus' s of 225 days ; 
Mercury is 35,393,000 miles from the sun, and Venus 66,131,000. Then the 
following proportion holds good, or nearly so : — 

88 2 : 225 2 : : 35,393,000'3 : 66,131,000 3 

956. Kepler's laws have been verified by all the observations made since 
his time. They gave a wonderful impetus to the science, corrected many 
false notions, and enabled astronomers to arrive at new facts from facts al- 
ready known. After many attempts and failures, the third law was finally 
reached on the 8th of May, 1618. "Perhaps", says Playfair, "philosophers 
will agree that there are few days in the scientific history of the world which 
deserve so well to be remembered." 

957. Aspects of the Planets. — By the Aspects of the 
planets are meant their positions in their orbits relatively 
to each other. The aspects most frequently alluded to are 
Quadrature, Conjunction, and Opposition. 

trate this law with Fig. 332. "What follows from this law with respect to the velocity 
of a planet ? In what part of its orbit does a planet move with accelerated velocity? 
In what, with retarded? Show the difference in the case of the Earth. What is the 
cause of this difference of ve\ocity? 955. State Kepler's Third Law, and gire an ex- 
ample. 956. What is said of Kepier's Laws and the estimation in which the third is 
field by philosophers ? 957. What is meant by the Aspects of the planets ? What 



ASPECTS OF THE PLANETS. 



379 



958. Quadrature. — Two heavenly bodies are said to be 
in Quadrature when they are 90 degrees apart ; that is, 
when, if either were placed on the other's orbit at a point 
corresponding to its position on its own, the arc between 
them would subtend an angle 
of 90° at the focus. Thus, 
in Fig. 333, E represents the 
Earth, and Q Mars in quad- 
rature. In almanacs and 
astronomical works, quad- 
rature is denoted by 
sign d. 

959. Conjunction. — Hea- 
venly bodies are said to be in 
Conjunction when they are 
seen in the same quarter of 
the heavens. Thus, in Fig. 
333, Venus (V), the Sun (S), 
and Mars (N), are in conjunction, being in the same direc- 
tion from the Earth (E). Conjunction is denoted by the 
sign 6 . 

Conjunctions are of two kinds, Superior and Inferior. A planet is in Su- 
perior Conjunction when it is in conjunction on the opposite side of the sun 
from the Earth, as Venus at W, and Mars at N. A planet is in Inferior Con- 
junction when it is in conjunction on the same side of the Sun as the Earth is, 
as Venus at V. It is evident that the superior planets can never be in infe- 
rior conjunction. 

960. Opposition. — Two heavenly bodies are said .to be 
in Opposition when they are in directly opposite quarters 
of the heavens. Thus, in Fig. 333, Mars at M and the Sun 
(S) are in opposition, because relatively to the Earth (E) 
they lie in opposite directions. The inferior planets never 
appear in opposition. Opposition is denoted by the sign <p. ' 




are the three principal aspects? 958. When are two heavenly bodies said to be in 
Quadrature ? Illustrate with the Figure. 959. When are heavenly bodies said to be 
in Conjunction ? Illustrate with Fig. 333. How many kinds of conjunctions are 
there? When is a planet said to be in Superior Conjunction? In Inferior Conjunc- 
tion? To what planets is inferior conjunction confined? 960. When are two heav- 
enly bodies said to be in Opposition ? Illustrate with Fig. 333. 961. What is meant 



3 80 ASTRONOMY. 

961. Transits. — The passage of an inferior planet across 
the Sun's disk is called its Transit. In Fig. 333, Yenus at 
V is making her transit. 

A transit can take place only when a planet is in inferior conjunction. 
But, as the orbits of the planets are in different planes, there may be inferior 
conjunctions without any transit. Yenus may be seen from the Earth in the 
same quarter as the Sun, and yet lie out of the plane which connects the cen» 
tres of the Sun and the Earth. 

962. Occultation. — When a planet or star is hid from 
the view of an observer on the Earth by the interposition 
of some other heavenly body, it is said to suffer occultation. 

9G3. Real and Apparent Motions. — An observer at 
the Sun would see all the planets moving around him from 
west to east with perfect regularity and always in the same 
direction. He would see their Real Motions. An ob- 
server on the Earth sees only their Apparent Motions, and 
these are so irregular that one might almost fancy the 
bodies in question wandering through space without any 
fixed law to direct their course. They are seen at one 
time moving from west to east, at another stationary, and 
again pursuing a retrograde course from east to west. 

The reasons of this are — 1. We are 91,430,000 miles from their centre of 
motion. 2. "We are ourselves moving, both round the sun and round the 
Earth's axis. Unconscious of these motions, we intuitively attribute the 
changes of direction produced by them to the motions of the orbs around us ; 
just as a person on a boat, when it begins to move, seems to be at rest him- 
self, and to see the wharf receding from him. 

964. Are the Heavenly Bodies inhabited? — This 
question is often asked, but can not be answered. No evi- 
dences of inhabitants have ever been discovered, even in 
the Moon, which is the nearest to us of all the heavenly 
bodies ; nor can there be any till great improvements have 
been made in the telescope. Nothing, however, seems to 
be created without an object; and, humanly speaking, it 
would be strange if of all the orbs which Omnipotence has 



by a Transit? Show the difference between a transit and inferior conjunction. 
962. When is a planet or star said to suffer occultation ? 963. What is the difference 
between the Real and the Apparent Motions of the planets? Describe the apparent 
motions. What causes are assigned for their irregularity ? 964. What is said with 



MERCURY. 381 

called into being our little world were the only one peopled 
by intelligent creatures. 

If the planets are inhabited, it must be by creatures constituted very dif- 
ferently from the human race. Surrounded by entirely different circum- 
stances as regards temperature, gravity, atmosphere, &c, the inhabitants of 
the different planets must be distinct races each from every other. Yet who 
can doubt that the same Infinite Wisdom that has adapted us to our sphere 
-jould as easily adapt them to theirs ? 

We proceed to consider the planets in turn. The char- 
acter annexed to the name is the mark by which the planet 
is denoted. 

965. Mercury ( ? ). — The nearest planet to the Sun is 
Mercury. Under favorable circumstances, Mercury may 
be seen at certain times of the year for a few minutes after 
sun-set or before sun-rise. At other times it keeps so close 
to the Sun as to be invisible, being lost in the superior 
brightness of his rays in the daytime, and setting and rising 
so nearly at the same time with him as to afford no oppor- 
tunity of observation. 

To the naked eye Mercury looks like a star of the third 
magnitude, twinkling (unlike the other planets) with a pale 
rosy light. Viewed through the telescope, it exhibits sim- 
ilar phases or changes of appearance to those of the moon 
(from full to new) ; this is because we see more of its en- 
lightened side at one time than another. 

The solar heat received at Mercury is seven times as great as that of the 
Earth, — a temperature more than sufficient to make water boil. Mercury's 
light is also seven times as intense as ours, and the Sun seen from this planet 
would look seven times as large as it does to us. No permanent spots are 
visible either on Mercury or Venus, whence it has been supposed that we do 
not see the surfaces of these planets, but only their atmospheres loaded with 
clouds, which may serve to mitigate the otherwise intense glare of the sun. 
A German astronomer, however, at the commencement of the present cen- 
tury, observed what he regarded as a number of mountains on the surface 
of Mercury, one of which he computed to be over 10 miles in height. 



respect to the heavenly bodies' being inhabited? 905. What is the nearest planet tc 
the Sun? When is Mercury visible ? What makes it invisible at other times? How 
does Mercury look to the naked eye ? Viewed through the telescope, what phases 
does it present ? How do tho solar heat and light received at Mercury compare with 
ours? Are any permanent spots visible on Mercury or Venus? To what supposi- 
tion has this fact led ? What was observed on Mercury by a German astronomer ? 



382 ASTRONOMY. 

Mercury's orbit deviates from a circle much more than that of any other 
planet, the asteroids excepted. This circumstance, combined with the in- 
clination of the plane of its equator to that of its orbit, must produce a 
great variety of seasons, and extreme changes of temperature. 

966. Venus ($). — The second planet from the Sun is 
Venus. On account of its nearness, it appears larger and 
j. lore beautiful to us than any other member of our plane- 
tary system. So bright is Venus that it is sometimes visi- 
ble at mid-day to the naked eye, and in the absence of the 
Moon casts a perceptible shadow. 

Being an inferior planet, Venus is never in opposition 
to the sun, and is always below the horizon at midnight. 
During part of the year, it rises before the Sun, and ushers 
in, as it were, the day ; when appearing at this time, the 
ancients styled it Phosphor or Lucifer (the light-bearer), and 
we call it the Morning Star. During the rest of the year, 
it rises after the Sun ; it was then styled Hesperus or Ves- 
per by the ancients, and is distinguished by us as the Even- 
ing Star. 

In size and density, Venus ranks a little below the Earth. The great 
inclination of the plane of its equator to that of its orbit must make a great 
difference in the relative length of day and night, and subject its polar 
regions to extreme changes of temperature. 

Venus's heat and light are twice as great as ours. So intense is its 
brightness that variations in its surface (if indeed its surface is not hid from 
us by a cloudy atmosphere) for the most part escape detection, every portion 
of the disk being flooded with light. Yet spots have occasionally been seen 
on its surface, and mountains have been observed having an estimated height 
of from 15 to 20 miles. Venus's phases, when viewed through the telescope, 
are similar to those of Mercury and the Moon ; but it never appears ex- 
actly full, being invisible at the time when this phase would otherwise be 
presented. 

967. The Earth (0). — The third planet from the Sun 
is the Earth, which we inhabit. ■ 

The form of the Earth is that of an oblate spheroid, — 



What is stated with respect to Mercury's seasons ? 966. What is the second planet 
from the Sun ? How does it look to us, and why ? What proofs have we of Venus's 
brightness? When is Venus called the Morning, and when the Evening Star ? How 
does the size of Venus compare with that of the Earth ? How do its heat and light 
compare with onrs ? What have been observed on Venus's surface ? What phases 
.Joes she present ? 967. What is the third planet from the Sun ? What is the form 



THE EARTH. 383 

that is, a sphere flattened at the poles like an orange. Its 
equatorial diameter is 7925.8 miles, and its polar diameter 
26£ miles less. The circumference of a sphere is a little 
more than three times as great as its diameter ; the dis- 
tance round the earth, therefore, is about 25,000 miles. 

The Earth is so large that its rotundity is not apparent to a person stand- 
ing on its surface. We know it to be round, however, in several ways. 
1. Navigators have sailed round it. By keeping the same general direction, 
east or west (as far as the land would allow), they have arrived at the place 
of starting. 2. The highest part of a vessel approaching in the distance is 
seen first, the lower part being obscured by the rotundity of the earth's sur- 
face. If the earth were a plain, we should seethe hull before the top-mast, 
inasmuch as it is larger. 

968. Motions. — The Earth turns on its axis once in 24 
hours. This is called its Diurnal Motion. Constantly bring- 
ing new points of the surface before the sun, and withdraw- 
ing others from its beams, this motion produces the suc- 
cession of day and night. 

The circumference of the earth being 25,000 miles, and a complete revo- 
lution being made in 24 hours, it follows that every point on the equator must 
revolve at the rate of a little over 1,000 miles an hour. As we go towards 
the poles, circles drawn round the earth parallel to the equator diminish in 
length, and points situated on them will consequently move with less ve- 
locity. At the poles there is no diurnal motion at all. 

969. The Earth has also an Annual Motion, — about the 
Sun. Its orbit, like that of the other planets, is elliptical, 
but does not deviate much from a circle. Its perihelion is 
3,000,000 miles nearer the Sun than its aphelion ; conse- 
quently at the former point, other things being equal, it 
receives more heat than at any other part of its orbit. 

The Earth reaches her perihelion on the 1 st of January every year. Hence 
our winter is somewhat milder than that of the southern hemisphere ; while 
the Sun at that period of a southern summer is perceptibly hotter than the 
summer sun at corresponding latitudes in the north. The heat in the inte- 



of the Earth ? What is its equatorial diameter ? Its polar diameter ? Its circum- 
ference ? Why do we not see the roundness of the Earth ? How do we know it to 
be round ? 968. What is meant by the Earth's Diurnal Motion ? What does it pro- 
duce? What is the velocity of the diurnal motion at the equator? At the poles? 
At intermediate points ? 969. What is meant by the Earth's Annual Motion ? What 
is the shape of its orbit ? When does the Earth receive the most solar heat, and 
why ? How do the northern and southern winter and summer compare ? Explain 



384 ASTRONOMY. 

rior of Australia at the time the Earth reaches her perihelion, is said to be 
more intense than any known even about the equator. Yet the difference of 
distance is so small compared with the whole, as not very materially to affect 
the Earth's temperature ; nor has it anything to do with the change of sea- 
sons, as we shall presently see. 

970. The Earth's orbit is about 575,000,000 miles in 
length ; and to get round it in 365 days 5 h " 48 ra - 46 s (which 
is the period of its revolution and constitutes our year), it 
must travel over 65,000 miles an hour. 

Though we are constantly moving with this great velocity, we are uncon- 
scious of it. This is because we have never known what it is to be at abso- 
lute rest ; and again, the motion is perfectly easy and regular, there being no 
obstructions in the way to make us sensible of it. 

971. The Earth in Space. — Space extends infinitely on 
all sides of the Earth, studded with stars at different dis- 
tances. To us, however, the stars appear equally distant, 
and seem to lie on the inner surface of a vast hollow sphere, 
at the centre of which we are placed. For purposes of defi- 
nition and description, it is often convenient thus to allude 
to the firmament ; and the expressions " celestial arch ", 
" concave surface of the heavens ", are used for the pur- 
pose, — not to denote any real objects, but the apparent 
arch or concave surface that we may conceive to be thrown 
around us. 

972. Horizon, Zenith, Nadir. — The Sensible Horizon is 
the line that bounds the view, — that is, where earth and 
sky appear to meet. To an observer on the ocean, or on 
a vast plain where there is nothing to obstruct the view, 
this line is always a circle. The plane passing through the 
sensible horizon, and infinitely extended through space, is 
called the Plane of the Sensible Horizon. 

The Rational Horizon is a plane passing through the 

Earth's centre, parallel to the plane of the sensible horizon. 

At the Earth these planes are separated by the distance between the cen- 

the cause. Is the Earth's temperature materially affected by this difference of dis- 
tance ? 970. With what velocity does the Earth travel round its orbit ? Why are 
we not sensible of moving? 971. What is meant by the expressions, "celestial 
arch", "concave surface of the heavens"? 972. What is the Sensible Horizon? 
What is the Plane of the Sensible Horizon ? What is the Rational Horizon ? What 



THE ECLIPTIC. 385 

tre and the surface, or 4,000 miles ; but so small is this distance compared 
with that at which the stars are situated that the two planes are regarded as 
striking the celestial arch at the same point. All heavenly bodies above the 
rational horizon at any given point are visible, and all below it invisible. 

973. The Poles of the Horizon are two points in the 
heavens equally distant from the circle that bounds the 
view. One of these, the point directly overhead, is called 
the Ze'-nith; the opposite point, directly beneath us, is 
sailed the Na'-dir. 

Every point on the Earth's surface has a horizon, zenith, and nadir of its 
own; and the horizon, zenith, and nadir of every point are constantly 
changing, owing to the revolution of the Earth on its axis. Hence, at night, 
new heavenly bodies are constantly coming into view in the east, while others 
are setting in the west. 

974. The Ecliptic. — Seen from the Sun, the Earth would 
appear to describe a circle round that luminary, among the 
fixed stars on the concave surface of the heavens. This 
circle corresponds with the apparent path of the sun as 
seen from the Earth, and is ealled the Ecliptic. 

The plaDe of the Earth's equator, extended till it meets 
the concave surface of the heavens, forms what is called 
the Celestial Equator, or the Equinoctial. The ecliptic and 
the equinoctial form an angle of 23° 27K', and this angle 
is called the Obliquity of the Ecliptic. The axis of the 
Earth, therefore, instead of being perpendicular to the 
plane of its orbit, is inclined to it at an angle of (90° — 23° 
27K') 66° 32V. 

975. The ecliptic cuts the equinoctial at two points, 
called Equinoxes, because when the sun appears at these 
points the days and nights are equal all over the world. 

The equinoxes are distinguished as Yernal and Autumnal. The Verna! 
Equinox is that point at which the sun crosses the equinoctial from south to 
north, which takes place in our spring. The Autumnal Equinox is the point 

is the distance between the two horizons at the Earth ? When they strike the celes- 
tial arch ? Which of the heavenly hodies are visible at any given point, and which 
invisible ? 973. What are the Poles of the Horizon ? What is the Zenith ? The 
Nadir ? What causes new heavenly bodies to keep coming into view at night and 
others to set ? 974. What is the Ecliptic ? What is the Celestial Equator, or Equi- 
noctial? What is the Obliquity of the Ecliptic? 975. What are the Equinoxes? 
Why are they so called? How are they distinguished ? What is the Vernal Equi- 

25 



386 ASTRONOMY. 

at which the sun crosses the equinoctial from north to south, — and this he 
does in our autumn. 

976. The Zodiac. — The Zodiac is a belt on the concave 
surface of the heavens, sixteen degrees in width, eight of 
which lie on each side of the ecliptic. It is divided into 
twelve Signs, of 30 degrees each. The zodiac is peculiarly 
interesting to us, because it is the region within which the 
apparent motions of the Sun, the Moon, and all the greater 
planets, are performed. 

The zodiac is so called from a Greek word signifying animal, because its 
signs were for the most part named after animals, of which the stars in each 
seemed to the ancients to be so grouped as to form rude outlines. Such 
groups of stars, which seem to be situated near each other because lying in 
the same direction from us, are called Constellations. Owing to what is 
known as the Precession of the Equinoxes, — that is, the sun's completing its 
revolution on the ecliptic every year before it reaches the same point of the 
heavens relatively to the fixed stars, — the signs of the zodiac do not now 
correspond in position with the constellations from which they were named. 
With the equinoxes, on which their position depends, they have retrograded 
30 degrees towards the west. The signs of the zodiac and the constellations 
of the zodiac must therefore be distinguished from each other. 

977. The names of the signs of the zodiac are given below in Latin and 
English, with the characters by which they are respectively denoted. They 
are given in their order, commencing at the vernal equinox. 

f Aries, the ram. 
tf Taurus, the bull, 
n Gemini, the twins. 
£p Cancer, the crab. 
SI Leo, the lion. 
TTJ2. Virgo, the virgin. 

978. The Change of Seasons. — It has been stated that 
the Earth is nearer the Sun at one period of its revolution 
than at another. The change of seasons, however, is en- 
tirely independent of this fact, and is produced by the sun's 
rays falling on a given point of the Earth's surface with 
different degrees of obliquity at different parts of its orbit. 

nox? What is the Autumnal Equinox? 976. What is the Zodiac? How is it di- 
vided ? What makes it peculiarly interesting to us ? From what is the zodiac so 
called ? What are Constellations ? How are the signs of the zodiac now situated 
relatively to the constellations from which they were named ? To what is this ow- 
ing? 977. Name the signs of the zodiac. 978. By what is the change of seasons pro- 



=£= Libra, the balance. 

nX Scorpio, the scorpion. 

£ Sagittarius, the archer. 

V3 Capricornus, the goat. 

£? Aquarius, the water-bearer. 

X Pisces, the fishes. 



THE CHANGE OF SEASONS. 387 

When the Sun is vertical, or directly overhead, its heat is 
most intense ; and the less its rays deviate from a vertical 
line in striking the surface, the more heat they impart to it. 
The angle at which the Sun's rays strike a given part 
of the Earth's surface keeps constantly varying, in conse- 
quence of the Earth's revolving with its axis always point- 
ing in the same direction. This is shown in Fig. 334. 

Fig. 334. 




At the vernal equinox (March 21st), the solar rays fall at the same angle 
on the northern hemisphere as on the southern, and it is spring in the 
former, autumn in the latter. Half the surface, from pole to pole, is illu- 
mined at a time, and day and night are of equal length all over the globe. 

As the Earth moves east with its axis pointing in the same direction, 
the Sun's rays no longer fall vertically on the equator, hut on places north 
of it. This continues till the summer solstice (June 21st), when the Sun is 
vertical to places 23° 27^' north ; his rays then extend 23° 27)^' beyond the 
north pole, and are shut out from the regions around the south pole to the 
same distance. It is now summer in the north, and winter in the south. 

duced? When is the Sun's heat most intense? Why does the angle at which the 
Sun's rays strike a given part of the Earth's surface keep varying ? With the aid 
of Fig. 334, describe the position of the Earth and the circumstances attending 



388 ASTRONOMY. 

The Sun is never directly overhead to any place farther north of the 
equator than 23° 27)^ '. As the Earth continues her course eastward, it 
becomes vertical to places more and more to the south, and by the 22d of 
September, or thereabouts, it is vertical to the equator just as it was six 
months before. This is the period of the autumnal equinox. The Earth 
again presents a full side from pole to pole to the Sun, and the days and 
nights are once more equal. We have now the southern spring and the 
northern autumn. 

From this point, the solar rays become more and more oblique in the 
north and fall vertically on places farther and farther south, till the same 
limit of 23° 27^ ' is attained, which takes place about December 21st, and 
marks the northern winter and the southern summer. Beyond this limit 
the Sun is never directly overhead. As the Earth keeps on her course, his 
vertical rays fall on latitudes nearer and nearer to the equator, till finally 
on the 21st of March places on the equator have the Sun in their zenith as 
they had six and twelve months before. 

979. The explanation just given shows that there are 
two points of the ecliptic in which the Sun is about 23£ de- 
grees from the equator, and from which he seems to turn 
back towards that line. These points are called Solstices 
{standing-points of the Sun), because the Sun appears to 
stand still for several days at the same place in the heav- 
ens before taking an opposite direction. The solstice 
reached in June is called the Summer Solstice ; that in De- 
cember, the Winter Solstice. 

980. Circles on the Earth's surface about 23| degrees 
north and south of the equator form the limits beyond 
which the Sun's rays are never vertical. These circles are 
called Tropics (from a Greek word meaning to turn), be- 
cause on reaching them the vertical rays turn back towards 
the equator. The northern tropic is called the Tropic of 
Cancer, because when the Sun reaches this line he is seen 
from the Earth in the sign Cancer, as will be apparent from 
Fig. 334. For a similar reason the southern tropic is called 
the Tropic of Capricorn. 

981. It appears from Fig. 334 that from March 21 to September 22 the 
north pole is constantly illuminated and the south pole in darkness, notwith- 
standing the revolution of the Earth on its axis ; while from September 22 

it, at March 21. At June 21. At September 22. At December 21. 979. What are 
the Solstices ? Why are they so called ? How are they distinguished ? 980. What 
are the Tropics? Whence is their name derived? What is the northern tropic 






THE MOON. 389 

to March 21, darkness reigns at the north pole and the south pole enjoys 
continual light. At the summer solstice there is a space of 23y 2 degrees 
about the north pole on which the Sun does not set, and at the winter sol- 
stice a corresponding space about the south pole. The lines that bound 
thesfc regions are called the Polar Circles. The one near the north pole is 
called the Arctic Circle ; that near the south pole, the Antarctic Circle. 

982. If, instead of being inclined, the Earth's axis were perpendicular to 
the plane of its orbit, the regions on the equator would have the Sun con- 
stantly in their zenith, day and night would always be equal over the whole 
globe, there would be no variety of seasons, and a given place would have 
about the same temperature from one year's end to another. Something of 
this kind must be the case on the planet Jupiter, whose axis is nearly per- 
pendicular to the plane of its orbit. On the other hand, the more the axis 
of a planet is inclined, the greater are the extremes of temperature incident 
to its several seasons. 

983. The Moon (#). — The Earth is attended by one 
satellite called the Moon, — a beautiful orb which c rules the 
night ' with its gentle brilliancy, produces in part the tides, 
and sensibly affects the Earth's motions by its attraction. 

984. Size. — The Moon's diameter is 2,153 miles, but its 
apparent size is almost equal to the Sun's, in consequence 
of its nearness to our planet. Its density is not much more 
than one-half that of the Earth, and it contains about one- 
eightieth as much matter. 

985. Motions. — The Moon is 240,000 miles from the 
Earth, and revolves about the latter so as to reach the same 
point relatively to the fixed stars in 2 V days, 7 hours, 43 
minutes. To reach the same point relatively to the Sun 
requires 29 days, 12 hours, 44 minutes, since the Earth 
has itself meanwhile advanced in its orbit. — When nearest 
the Earth, the Moon is said to be in her Per'-i-gee, and 
when farthest from it in her Ap'-o-gee. 

The terms perigee and apogee (which mean near the Earth and away from 
the Earth) are also applied to the apparent position of the Sun. When the 
Earth is at its perihelion, the Sun is said to be in perigee ; and when the 
Earth is at its aphelion, the Sun is in apogee. 

called, and why ? The southern ? 981. What are the Polar Circles ? What is the 
one near the north pole called ? That near the south pole ? 9S2. If the Earth's axis 
were perpendicular to the plane of its orbit, what would follow ? What is said of 
Jupiter? 9S3. By what is the Earth attended ? 984. How great is the Moon's diam- 
eter? Its density? Its mass? 985. How far is the Moon from the Earth ? What 
if the period of her revolution ? When is the Moon said to be in perigee ? In ap- 



390 ASTRONOMY. 

The Moon also turns on its axis in exactly the same time 
that it takes to revolve round the Earth, and in the same 
direction. The consequence is that she always presents the 
same side to the Earth. Nearly one-half of our fair- at- 
tendant we never see, and to the inhabitants of half her 
surface, if she has any, we are invisible. 

986. Phases. — The Moon is non-luminous, and shines 
only by the reflected light of the Sun; hence the hemi- 
sphere presented to the Sun is bright, while the opposite 
one is dark. As the Sun, Moon, and Earth are constantly 
taking different positions relatively to each other, the por- 
tion of illuminated lunar surface presented to us is as con- 
stantly changing. Hence arise what are called the Phases 
of the Moon. 

When new, the Moon lies between the Earth and the Sun, near a line con- 
necting their centres. Her dark side is then towards us, and she is invisible. 
Soon, however, she gets so far east of the Sun as to appear in the west 
shortly after his setting. A bright crescent then becomes visible on the side 
nearest the Sun, the rest of her circular disk being just discernible, not by 
sun-light directly received, but by sun-light reflected from the Earth to the 
Moon, and by her reflected back to us. The crescent gradually grows larger, 
until, when the Moon is 90 degrees from the Sun, or in quadrature, half her 
disk is illumined. She is then said to be in her First Quarter. 

Each succeeding night now finds the enlightened portion larger and 
larger, and the Moon is said to be gibhous. At last she reaches a point at 
which she is again almost in a line with the Sun and the Earth, but this time 
the Earth is in the middle. The Moon rises in the east as the Sun sets in the 
west ; the whole of her enlightened hemisphere is therefore turned towards 
us, and she is said to be full. 

After this the Moon again becomes gibbous, and we see less and less of 
her enlightened surface, till at length half of her disk is dark, when she is 
said to be in her Third Quarter. Advancing beyond her third quarter, she 
wanes still further to a crescent, and at length on arriving in conjunction 
with the Sun disappears entirely, — to go through the same phases again as 
she makes another revolution in her orbit. 

987. To the inhabitants of the Moon, if any there be, the Earth presents 
the same phases that the Moon does to us, but in reversed order. When the 
Moon is new to us, the Earth is full to them, — a splendid orb, thirteen times 

ogee ? When is the Sun said to be in perigee ? In apogee ? How long is the Moon 
in turning on her axis ? What is the consequence ? 986. What is said of the Moon's 
light? What causes her to present different phases to the Earth? Describe the 
phases successively presented. 987. What phases does the Earth present to the 



THE MOON. 391 

as large as the full Moon. When she is in her first quarter, the Earth is in 
her third quarter, &c. 

988. The Moon has either no atmosphere at all, or one 
exceedingly rare, and not extending more than a mile from 
its surface. Hence it must be destitute of water, for any 
liquid on its surface would long since have been dissipated 
by the heat of the lunar days, there being no atmospheric 
pressure to check evaporation. If there were any water 
on the surface of the Moon, clouds would certainly be ob- 
served at times dimming its face. 

989. Viewed through a telescope, the surface of the Moon appears ex- 
ceedingly rough, covered with isolated rocks, deep valleys, yawning chasms, 
craters of extinct volcanoes, in some cases more than 100 miles in width, and 
lofty mountains, several of which are from three to four miles high and cast 
their shadows a great distance over the rugged plains. Every thing is deso- 
late in the extreme. Several of the earlier astronomers thought that they 
discerned volcanoes in a state of eruption ; but later observers are of the con- 
trary opinion, attributing the peculiar brightness of the supposed volcanic 
summits to phosphorescence, or superior reflective properties. 

Names have been given to the various mountains and spots visible on the 
Moon, and a map has been prepared of the whole side presented to us, which 
has been pronounced " vastly more accurate than any map of the Earth we 
can yet produce." — The great telescope of the Earl of Rosse shows with dis- 
tinctness every object on the lunar surface that is 100 feet in length. It has 
brought to light, however, no signs of life or habitation. 

990. Maes ( $ ). — Mars, the fourth planet from the Sun, 
is 4,920 miles in diameter. Its day is of nearly the same 
length as ours, its year about twice as long. The incli- 
nation of its axis to the plane of its orbit does not differ 
much from the Earth's, and its seasons are therefore simi- 
lar to ours. It is surrounded by an atmosphere of mod- 
erate density. Mars has two moons, discovered in 1877. 

Mars is easily distinguished in the heavens by his red fiery light, which 
is supposed to owe its color to the soil from which it is reflected. The tele- 
scope distinctly shows continents of a dull red tinge, like that of sand-stone, 

Moon ? 988. What is said of the Moon's atmosphere ? Why is the Moon sup- 
posed to be destitute of water ? 989. How does the Moon look, when viewed through 
a telescope ? What is now thought respecting the supposed volcanic eruptions for- 
merly observed ? How high objects does the Earl of Kosse's telescope distinctly 
6how? 990. Which is the fourth planet from the Sun ? What is the length of its 
diameter? Its day ? Its year ? How do its seasons compare with ours ? How may 
Uars be distinguished ? What does the telescope show ? What are seen about the 



892 THE ASTEROIDS 

washed by seas of a greenish hue. Bright white spots are seen about the 
poles, which are no doubt occasioned by the reflection of the sun's light from 
the snow and ice collected there. It is observed that as each pole is turned 
towards the sun the spots about it diminish in size, owing to the melting of 
the snow by the solar heat. 

991. The Asteroids. — The Asteroids are so small that, 
with the exception of one or two which have been seen 
without a telescope, they are invisible to the naked eye. 
Vesta, the largest, is but 228 miles in diameter, and many 
of the smaller ones are less than 50. Pallas and others 
are supposed to have dense atmospheres. It has been 
suggested that the Asteroids may be the fragments of one 
large planet, originally revolving between Mars and Ju- 
piter, but shattered by some tremendous catastrophe. The 
fact, however, that the orbits have no common point of 
intersection, makes this theory improbable. There are, 
no doubt, many asteroids yet to be discovered. 

The Asteroids are comparatively so diminutive that the force of gravity 
on their surfaces must be very small. A man placed on one of them would 
spring with ease 60 feet high, and sustain no greater shock in his descent 
than he does on the earth from leaping a yard. On such planets giants may 
exist ; and those enormous animals which here require the buoyant power 
of water to counteract their weight, may there inhabit the land. 

992. Jupiter (U). — Next to the asteroids is Jupiter, 
the largest of the planets, which exceeds the Earth in bulk 
nearly 1,300 times. Its revolution round the Sun is per- 
formed in about 12 years, and that around its axis in less 
than 10 hours. Jupiter is attended by four satellites, which 
revolve about it from west to east. 

All of these satellites but one exceed our Moon in size. The largest would 
sometimes be visible to the naked eye as a very faint star, were it not lost in 
the superior brightness of its planet. Three of them are totally eclipsed 
during every revolution by the long shadow which the planet casts, and the 
fourth is very often eclipsed. The relation between their orbits and motions 
is such that for many years to come Jupiter will never be deprived of the 
light of all four at the same time. 

poles ? By what are tbey supposed to be caused ? 991. Are the Asteroids visible to 
tbe naked eye ? What is the length of their diameters ? What has been suggested 
respecting their origin ? What is stated with respect to the force of gravity on their 
6urface? 992. How does Jupiter rank in size? How does it compare in bulk with 
the Earth? What is the length of its year? Its day? By what is it attended ? 



SATURX. 393 

So large is Jupiter, and so short a time is it in revolving on its axis, 
that every point on its equator must turn at the rate of 450 miles a minute. 
The result is an immense centrifugal force at the equator ; and this is seen 
to have operated before the mass of the planet became hard, by flattening 
it very much at the poles. — Jupiter's disk is always crossed with a number 
of dark parallel belts, as shown in Fig. 331. They vary in breadth and 
situation, but are always parallel to the equator of the planet ; hence they 
appear to be connected with its rotation on its axis, and are no doubt pro- 
duced by disturbances in its atmosphere. 

992. Saturn (^). — Saturn, which is next to Jupiter in 
distance from the Sun, is also next to it in size, having a 
diameter of 71,900 miles, and consequently a bulk nearly 
750 times that of the Earth. Its day is not half so long 
as ours ; but it is 29^ of our years in making one complete 
revolution in its orbit. 

Saturn has eight moons, seven of which were known for sixty years 
before the eighth was discovered. The largest of them has a diameter 
about half as large again as our Moon. Saturn's disk, like Jupiter's, is 
frequently diversified with belts ; spots are of rare occurrence. An atmos- 
phere of considerable density is supposed to surround the planet. 

Saturn has a remarkable appendage, consisting of three bright, flat, 
thin rings, detached from each other but close together, encircling the 
planet at its equator, and revolving with it around its axis in about the 
same time in which the planet itself revolves. The whole breadth of these 
rings is 37,570 miles, while their thickness does not exceed 100 miles. The 
innermost one is less bright than the two beyond it, and is 9,760 miles 
from the surface of the planet. They are supposed to be made up of myr- 
iads of little satellites, moving independently, each in its own orbit, round 
the planet, — presenting the appearance of a bright ring where they are 
closely packed together, and a dim one where they are scattered. 

993. Uranus (W). — Uranus, the next planet to Saturn, 
revolves about the Sun in 84 of our years. There being no 
spots on its surface, we are unable to fix the period of its 
rotation on its axis. It is attended by four moons, which 
move from east to west (unlike the satellites of the other 

What is the size of the largest of these moons ? What relation subsists between their 
orbits and motions ? "What is the sbape of Jupiter ? What has caused the flattening 
at the poles ? With what is Jupiter's disk crossed ? To what are these belts to be 
attributed ? 992. What is the nest planet to Jupiter ? What is Saturn's diameter? 
How does its bulk compare with the Earth's? Its day? Its year? How maDy 
moons has Saturn? How is its disk diversified ? What remarkable appendage has 
Saturn ? Describe its rings. 993. What Is the next planet to Saturn ? Wbat is tho 
length of the year of Uranus ? Its day ? By what is it attended ? How do its light 

17* 



394 ASTRONOMY. 

planets) in orbits nearly perpendicular to that of the planet. 
The solar heat and light of Uranus are only 3^ of ours. 

994. Neptune (¥). — Neptune, the most remote planet 
of the solar system, is invisible to the naked eye. Seen 
through the telescope, it looks like a star of the eighth 
magnitude. The diameter of Neptune is 36,600 miles, 
which is 3,600 more than that of TJranus. Its rev- 
olution around the Sun is performed in about 165 of our 
years. Neptune has at least one moon, distant from it about 
as far as ours is from us. 

The discovery of Neptune is one of the greatest triumphs of which sci- 
ence can boast. Comparing observations on Uranus, while it was still 
thought to be the most distant member of the solar system, astronomers 
found certain 'perturbations or irregularities, in its motions, which could be 
accounted for only on the supposition that there was some unknown planet 
beyond it by whose attraction it was affected. Le Verrier thoroughly inves- 
tigated the subject, and even went so far as to compute the size and distance 
of the suspected planet, and to predict in what part of the heavens it would 
be found at a given date. A letter from the French astronomer, embracing 
the results of his calculations, reached Berlin, September 13, 1846 ; and that 
very evening, sweeping the heavens with his powerful telescope, according 
to Le Verrier's instructions, Dr. Galle discovered what was apparently a star 
of the eighth magnitude not laid down on his chart, but was proved by its 
change of place on the following evening to be a planet. — It is just to add 
that Adams, an English astronomer, had, about the same time with Le Ver- 
rier, made similar calculations, and with nearly the same result. 

995. Real and Apparent Position op the Heavenly 
Bodies. — We seldom see the heavenly bodies in their real 
position. This is owing to two causes, — Refraction and 
Parallax. 

996. Effect of Refraction. — Refraction, which has been 
explained in the chapter on Optics, bends rays of light en- 
tering our atmosphere from a rarer medium, and causes the 
body from which they proceed to appear higher than it 
really is. The Sun is thus made visible a few moments be- 
fore he actually rises and after he sets. The effect of re- 

and heat compare with ours ? 994. What is the most remote planet of the solar sys- 
tem? How does Neptune look, when seen through the telescope ? What is its diam- 
eter f What is the period of its revolution ? How many moons has Neptune ? Give 
an account of the circumstances under which Neptune was discovered. 995. Why do 
we not see the heavenly bodies in their real position ? 996. What is the effect of re 



PARALLAX. 395 

fraction is greatest when a body is on the horizon, and 
diminishes as it ascends towards the zenith, at which point 
it entirely disappears. 

997. Effect of Parallax. — A planet seen from different 
points of the Earth's surface appears to lie in different 
positions. This is evident from Fig. 335. 

The planet C to an observer ^. „„- 

dt A seems to lie at F ; to one * 

at B it appears to lie at D. To / — ^r— . ^_____^ „ 

avoid the inconsistencies which ( E L - ' =*'-"-~:" 

would otherwise exist in obser- x ^_^ »j» 

vations made at different places, 

the centre of the earth is taken as a standard point ; and the true position 
of a heavenly body is that point of the celestial arch which would be cut 
by a line connecting the centre of the Earth with the centre of the body in 
question, infinitely produced. 

Parallax is the angle made by a line from a heavenly 
body to the Earth's centre and another line from the same 
body to the eye of an observer. 

It is evident that, the nearer a heavenly body is, the greater is its parallax 
The fixed stars are so remote that they have no appreciable parallax. The 
Earth, if visible to them, would be nothing more than a minute point of light. 
— The parallax of a heavenly body is greatest when it is on the horizon. At 
the zenith it would be nothing, because from that point the lines to the ob- 
server's eye and the centre of the Earth would coincide. 

998. Eclipses. — By an Eclipse of the Sun or Moon is 
meant its temporary obscuration by the interposition of 
some other body. An eclipse is called Total, when the 
whole disk is obscured ; and Partial, when only a portion 
is darkened. 

999. An eclipse of the Sun is caused by the Moon's get- 
ting between it and the Earth, and intercepting its rays. 
This can happen only at new Moon, because, when between 
us and the Sun, the Moon must present to as her unenlight- 
ened side. 

fraction ? 997. How does a planet seem to lie, when observed from different parts of 
the Earth's surface ? Illustrate this with Fig. 335. What is the true position of a 
heavenly body ? What is Parallax ? What is said of the parallax of the fixed stars ? 
What would be the effect of refraction and parallax on the apparent position of a 
body in our zenith ? 998. What is an Eclipse ? When is an eclipse called Total, and 
When Partial ? 999. What causes an eclipse of the Sun ? When alone can this hap- 



396 ASTRONOMY. 

If the Moon's orbit lay in the same plane as the Earth's, she would eclipse 
the Sun every time she became new ; but, as her orbit is inclined to the 
ecliptic at an angle of more than 5 degrees, her shadow may fall above or 
below the Earth at the time of her change. 

When the Moon intervenes between the Sun and the Earth at such a dis- 
tance from the latter as to make her apparent diameter less than the Sun's, a 
singular phenomenon is exhibited. The whole disk of the Sun is obscured, 
except a narrow ring around the outside encircling the darkened centre. 
This is called an Annular Eclipse, from the Latin annulus, a ring. 

1000. An eclipse of the Moon is caused by the Earth's 
getting between it and the Sun. This can take place only 
at full Moon, because when the Earth is between the Sun 
and the Moon the latter must present her enlightened side 
to the Earth. 

Non-luminous itself, when cut off from the solar rays, the Moon must be- 
come invisible. There is this difference between an eclipse of the Sun and 
the Moon. In the former, the Sun shines the same as ever, and its bright- 
ness is undiminished to those who are out of the Moon's shadow. When the 
Moon is eclipsed, on the other hand, she diffuses no light, and is dark to all 
within whose range of vision she is situated. — Solar eclipses occur more fre- 
quently than lunar. The greatest number of both that can take place in a 
year, is seven ; the smallest number, two ; the usual number, four. 

1001. When the Sun is totally eclipsed, the heavens are shrouded in dark- 
ness, the stars make their appearance, the birds go to roost, the animals by 
their uneasiness testify their alarm, and all nature seems to feel the unnatu- 
ral deprivation of the light of day. It is not surprising that, when the cause 
of the phenomenon was unknown, it filled the minds of men with consterna- 
tion. Even at the present day barbarous nations regard eclipses as indica- 
tions of the displeasure of their gods. Columbus, on one occasion, when 
wrecked on the coast of Jamaica, and in imminent danger both of starvation 
and an attack from the Indians, saved himself and his men by taking advan- 
tage of this superstitious feeling. From his acquaintance with astronomy, 
he knew that an eclipse of the Moon was about to take place ; and on the 
morning of the day, summoning the natives around him, he informed them 
that the Great Spirit was displeased because they had not treated the Span- 
iards better, and would shroud his face from them that night. When the 
Moon became dark, the Indians, convinced of the truth of his words, hastened 
to him with plentiful supplies, praying that he would beseech the Great 
Spirit to receive them again into favor. 

pen ? Why is not the Sun eclipsed every time the Moon becomes new ? What is an 
Annular Eclipse ? 1000. By what is an eclipse of the Moon caused ? When can this 
take place ? What difference is mentioned between an eclipse of the Sun and the 
Moon ? Which occurs more frequently ? What is the usual number in a year ? 
1001. Describe the appearance of things during a total eclipse of the Sun. How do 
barbarous nations regard eclipses ? How did Columbus once save himself and his 



COMETS. 397 

1002. Comets. — Comet is derived from a Greek word 
meaning hair ; and the term is applied to a singular class 
of bodies belonging to the solar system, from which long 
trains of light, called tails, spread out like hair streaming 
on the wind. They differ very much in appearance ; but, 
for the most part, they consist of a nucleus, which is a very 
bright spot, apparently denser than the other portions ; an 
envelope, which is a luminous fog-like cover surrounding 
the nucleus ; and a tail, which appears to be an expansion 
of the envelope produced by solar heat. 

The tails of different comets differ greatly in shape and extent. In some 
this appendage is entirely wanting ; in others it has been found to extend 
120,000,000 miles. Several tails have been exhibited at the same time ; the 
comet of 1744 threw out no less than six, like an enormous fan, over the 
heavens. Even in the same comet the tail keeps changing, being largest 
when near the Sun and diminishing as it recedes from that body. — The tail 
lies on the opposite side of the nucleus from the Sun, — behind it, when ap- 
proaching its perihelion, and preceding it when retiring from that point. 

1003. Constitution. — The matter of which comets are 
composed must be an exceedingly thin gas or vapor. 

This is shown by the fact that comets have on different occasions passed 
very near the planets, yet have never caused any irregularities in their 
motions, while their own motions have been materially affected. The tail, 
in particular, must be exceedingly rare, perhaps not weighing more than 
a few ounces, even when most extensive. Faint telescopic stars have been 
seen through the envelope, without any diminution of brightness. Comets 
shine chiefly, if not altogether, with light borrowed from the Sun ; they 
gradually become dimmer as they leave this luminary, and finally vanish 
from the loss of its light while they still have a sensible magnitude. 

1004. Orbits, Velocity. — The orbits of the comets are 
either ellipses, parabolas, or hyperbolas. 

If ellipses, they generally deviate very much from a circle, being length- 
ened out an immense distance in proportion to their breadth. Comets that, 
move in elliptical orbits return after a series of years ; those that move in 
parabolas or hyperbolas never reappear, but after wheeling about the Sun 
dash off into the remote regions of space, perhaps to visit other systems. 

Some comets at their perihelion pass very close to the Sun. The one that 

men ? 1002. What are Comets ? Of what do they consist ? What is said of the tails 
of different comets? In the case of the same comet, what change takes place in the 
tail ? How does the tail lie ? 1003. Of what kind of matter must comets be com- 
posed ? How is it proved that the matter of comets must be exceedingly rare ? 
1004 What shape are the orbits of comets ? In what case will the cornet return ? 



398 ASTRONOMY. 

appeared in 1843 almost grazed its surface, approaching so near it that the 
solar disk must have appeared 47,000 times larger than it looks to us, and 
the heat received must have been twenty-five times greater than that re- 
quired to melt rock-crystal. 

1005. When near the Sun, comets move with incredible 
velocity, — sometimes at the rate of over a million miles an 
hour. I 

1006. Number. — The exact number of comets can not 
be determined. Over seven hundred have been seen and 
enumerated. Multitudes have visited our system without 
being seen from the Earth, in consequence of reaching their 
perihelion in the day-time, or when the heavens were ob- 
scured by mists and clouds. Arago estimated the number 
that have appeared or will appear within the orbit of Ura- 
nus at 7,000,000 ; the same calculation extended to Nep- 
tune's orbit would make the number 28,000,000. 

1007. Comets were formerly regarded with superstitious terror as precur- 
sors of war, famine, and other misfortunes. In more modern times the fear 
of a collision made them formidable objects. This fear, however, has been 
dispelled by the discovery of their great rarity. A collision, however fatal 
it might be to the comet, would probably do little injury to a solid body like 
the Earth. 

The Fixed Stars. 

1008. The Fixed Stars are so called in contradistinction 
to the planets, because they maintain the same position rela- 
tively to each other, not because they are absolutely at rest. 
They all move about some fixed point in immense orbits, 
which it will take millions of years for them to complete. 
Shining by their own light and not by reflection, they are 
suns, and are probably each the centre of a system of its 
own. 

1009. Magnitudes. — Varying in size and situated at different distances 
from us, the stars are not all of the same brilliancy. They are divided into 
about twenty classes according to their brightness, and distinguished as stars 



How near did the comet of 1843 pass to the Sun ? 1005. What is the velocity of com- 
ets, when near the Sun ? 1006. What is the number of the comets ? What prevents 
as from seeing many that visit our system ? What was Arago's estimate ? 1007. How 
were comets formerly regarded ? How are they now looked upon? 1008. Why are 
the Fixed Stars so called ? 1009. How are the fixed stars classified ? What are Tel- 



THE FIXED STARS. 399 

of the First, Second, &c, Magnitude. The stars of the first six magnitudes 
are visible to the naked eye ; the rest are called Telescopic Stars, because 
seen only with the telescope. There are about 20 stars of the first magni- 
tude, 65 of the second, and 200 of the third ; but the number in the lower 
classes increases so rapidly as to be almost beyond enumeration. 

1010. Constellations. — For convenience of reference, the stars are divided 
into constellations, or groups, named after animals and other objects to which 
their outline bears some fancied resemblance. The twelve constellations 
of the zodiac have been already named ; there were 36 more laid off by the 
ancients in other parts of the heavens. The whole number has been increas- 
ed in modern times to 109. The stars in each constellation are distinguished, 
according to their magnitude, fiist by the letters of the Greek alphabet, 
then by those of the Eoman, and, when both are exhausted, by figures. 

1011. Distance. — The distance of the fixed stars is 
absolutely incredible. None of them can be less than 
19,200,000,000,000 miles from the Earth, while the greater 
part are far more remote. 

The recent improvements in telescopes have enabled astronomers to com- 
pute the distance of nine of the nearest stars. Sirius, the brightest of them, 
is found to be so far off that light, with a velocity of 185,000 miles a sec- 
ond, is twenty-one years in reaching us ; from the North Star it is over 
forty-eight years. The mind is lost in trying to comprehend such mighty 
distances ; and yet it will be remembered these are among the nearest stars. 

1012. Several remarkable facts are worthy of note in connection with the 
fixed stars. Some of them wane for a time, so as to be classed in a lower 
magnitude, and then resume their former brilliancy. Others, after vanishing 
entirely for a season, suddenly reappear ; these are called Periodical Stars. 

Many stars (more than six thousand), when viewed through a powerful 
telescope, are resolved into two stars, one of which is generally much 
fainter than the other. These are known as Double Stars. In some cases, 
the faint one may only appear to be near the bright one from lying in the 
same direction, and really be millions of miles behind it ; but there is gen- 
erally reason for supposing that the fainter luminary revolves about the 
brighter one in obedience to that same great law of gravitation which pre- 
vails in our own system.— Some stars, apparently single, are resolved into 
three, four, and even six, by the telescope. 

Many of the double stars are tinged with complementary colors. The 
larger one is orange-colored, the smaller blue ; or the one is red, and the 

escopic Stars ? How many stars are there of the first magnitude ? Of the second ? 
Of the third ? 1010. How are the fixed stars divided ? How many constellations 
were laid off by the ancients? How many have been added in modern times ? How 
are the stars in each constellation distinsruished ? 1011. What is the distance of the 
fixed stars ? What is the distance of Sirius ? Of the North Star? 1012. What are 
Periodical Stars ? What are Double Stars ? What relation seems to subsist between 
the brighter and fainter star? Into what are some stars resolved ? With what are 



4-00 ASTRONOMY. 

other green. Some of the single stars look blood-red, and others yellow, 
blue, and green. In several cases, changes of color have occurred. 

The size of several of the fixed stars has been calculated approximately. 
Their diameters are found to be enormous, — in one case, not less than 
200,000,000 miles. Sirius, " the dog-star", if set in the place of our Sun, 
would look 125 times as large as he, and give us 125 times as much light. 
Trillions of miles away, as it is, it dazzles the eye when seen through a 
"powerful telescope. 

1013. The Galaxy. — The Galaxy, or Milky Way, is a 
broad zone of light which stretches across the sky from 
horizon to horizon, maintaining the same position rela- 
tively to the stars. Examined through a powerful teles- 
cope, it is found to consist of stars, scattered by millions, 
like glittering dust, on the black ground of the heavens. 

1014. Nebtjlje. — Nebulae are faint cloud-like patches 
of light, visible some to the naked eye, and others only with 
the telescope. They differ in shape — a circular outline 
being the most common, — and are found in different quar- 
ters of the heavens. Many of them are resolved by the 
telescope into clusters of stars ; there are others, how- 
ever, (distinguished as True Nebulae), which are not so re- 
solvable, and which recent researches prove to be masses 
of incandescent gas, principally hydrogen and nitrogen. 

Lord Eosse's great telescope shows many of the nebulae to be simply 
remote star-clusters ; it makes others appear bright, without resolving them 
into stars ; and it calls up from the depths of space others which appear as 
faint, even to its mighty magnifying power, as those which it resolves ap- 
pear to the unaided eye. The milky way is itself a resolvable nebula, more 
distinct than the others because nearer to us. 

From the facts set forth we may conclude that the universe consists of a 
vast number of distinct clusters of worlds, separated from each other by im- 
mense intervals ; that the fixed stars, the milky way, our Sun and its sys- 
tem, form one of these clusters ; that some of the remote nebulae constitute 
other clusters, fainter or brighter according to their distance from us,— 
each composed of many different systems,— and having its members sep- 
arated as widely as our Sun is from the brother suns about him. 

How can the mind take in such mighty thoughts ! How can the heart 
refuse its homage to the great Creator of all these worlds ! 

many of the binary stars tinged ? What has been found with respect to the diame 
ters of some of the fixed stars? How would Sirius look, if set in the place of onr 
Sun ? 1013. What is the Galaxy ? 1014. What are Nebulae ? How do nebulae look 
through Lord Eosse's telescope ? What may we conclude from the facts set forth ? 



METEOROLOGY. 401 

CHAPTER XIX. 

METEOROLOG-Y. 

1015. Meteorology is the science which treats of the 
phenomena of the atmosphere. Among these are winds, 
clouds, fog, dew, rain, snow, and hail. 

Some of the phenomena of the atmosphere have been already described 
and explained in connection with the various subjects that have engaged our 
attention. 

1016. Wind. — Wind is air put in motion. 

The motion of the air is the result of changes constantly going on in the 
earth's temperature, in consequence of the alternation of day and night and 
the succession of the seasons. Those portions of the atmosphere that rest 
on the hotter regions of the earth become heated and rarefied, and rising 
leave a vacuum which is immediately filled by a rush of cooler air from the 
surrounding parts. Currents are thus produced, which we call winds. 

The direction of the wind is determined by various local causes, modified 
by the revolution of the earth on its axis. The latter, operating alone, would 
make it appear to blow uniformly from the east ; but the various projections 
on the earth's surface, and the unequal distribution of land and water (the 
latter of which is incapable of being heated to the same degree as the former), 
— these and other agencies constantly at work combine to give the wind dif- 
ferent directions at different places, and to make it vary at the same place. 

1017. Velocity. — The velocity of the wind is measured 
with an instrument called the An-e-mom'-e-ter. 

There are several kinds of anemometers. One of the 
best consists of a small windmill, with an index attached 
for recording the number of revolutions in a given time. 

It is found with the anemometer that a wind so slight as hardly to stir the 
leaves travels at the rate of 1 mile an hour ; a gentle wind, 5 miles in the 
same time; a brisk gale, 15 miles; a high wind, 30; a storm, 50; a hurri 
eane, SO ; a violent hurricane, 100. 

1018. Kinds. — There are three kinds of winds ; Con- 
stant, Periodical, and Variable. 

1015. What is Meteorology ? Mention some of the phenomena of the atmosphere. 
1016. What is Wind ? What puts the air in motion ? By what is the direction of the 
wind determined? 1017. How is the velocity of the wind measured? What is one 
of the best forms of the anemometer? How fast does a scarcely perceptible wind 
travel? A gentle wind ? Abriskgale? A storm? A hurricane? 1018. How many 

26 



402 METEOROLOGY. 

1019. Constant "Winds are those that blow throughout 
the year in the same direction. 

The most noted of these are the Trade Winds, which extend about 30 de- 
grees on each side of the equator, a zone of 6 degrees near the centre known 
as the Region of Calms being excepted. They blow uninterruptedly on the 
ocean from north-east to south-west in the northern hemisphere, and from 
south-east to north-west in the southern. The regions on the equator being 
more heated than the surrounding parts, the air resting on them is rarefied, 
and rising flows over the cooler masses towards the poles, while cold air from 
the latter rushes in below to supply its place. Were the earth stationary, the 
trade winds would be due north on one side of the equator, and due south on 
the other. The earth's diurnal revolution, however, from west to east, mod- 
ifies these directions so far as to make the north wind north-east and the 
south wind south-east. 

The trade winds are of great service to mariners, enabling them to make 
certain voyages (for instance, from the Canaries to the northern coast of 
South America) with great rapidity, and almost without touching a sail. The 
zone in which they prevail is noted for its transparent atmosphere, its uni- 
formity of temperature, and general peaceful aspect ; whence it has been 
called by the Spaniards " the sea of the ladies ". 

1020. Periodical Winds are such as blow regularly in 
the same direction at a certain season of the year or hour 
of the day. The monsoon, the simoom, and the land and 
sea breezes, are periodical winds. 

The monsoons are modifications of the trade winds, which sweep, some- 
times with great violence, over the Indian Ocean, Hindostan, and Farther 
India, and also the Gulfs of Guinea and Mexico, and the adjacent lands. 
For six months they blow from a certain quarter, and for the next six 
months from the opposite one, owing to the change in the sun's position. 

The simoom, originating in the deserts of Asia and Africa, is distinguished 
by its scorching heat and the fine sand it carries with it, raised from the 
parched surfaces it traverses. The simoom from the Desert of Sahara, 
sweeping over the intervening regions, finally reaches the northern shore of 
the Mediterranean, and is there called the Sirocco. — During the continuance 
of this hot and deadly wind, the animal and the vegetable creation droop 
with excessive exhaustion ; travellers on the desert save their lives only by 
throwing themselves down with their faces in the sand. 

Land and sea breezes are produced by the unequal heating of land and 

kinds of winds are there ? Name them. 1019. What are Constant Winds ? What 
are the most noted constant winds? Where do the trade winds blow, and in what 
direction ? Explain the origin of the trade winds. How do they benefit mariners ? 
What do the Spaniards call the region in which they prevail, and why ? 1020. What 
are Periodical Winds ? Mention some. Describe the monsoons. Where does the 
simoom originate, and by what is it distinguished ? What is the Sirocco ? What is 



WINDS. 403 

»rater. During the day, the land receives more heat than the adjacent ocean, 
the rarefied air in contact with it rises, and a gentle breeze sets in from the 
sea about nine in the morning, which gradually increases to a brisk gale in 
the middle of the day. About 3 p.-m. it begins to subside, and is followed in 
the evening by a land breeze, which blows freshly through the nigbt : for 
after sunset the land rapidly parts with its heat by radiation, and the air 
resting on it, becoming cooler than that on the ocean, rushes to supply the 
place of the latter when it rises in consequence of being rarefied. 

1021. Variable Winds are those which are irregular as 
to time, direction, and force, seldom continuing to blow for 
many days together. They prevail chiefly in the temperate 
and frigid zones, the winds of the torrid zone being for the 
most part constant or periodical. 

1022. Hurricanes. — Hurricanes are storms that revolve 
on an axis, while at the same time they advance over the 
earth's surface. 

Hurricanes are distinguished by their tremendous velocity and great ex- 
tent They are often 500 miles in diameter, and sometimes much more. In 
the southern hemisphere they always revolve in the same direction as the 
hands of a watch ; in the northern hemisphere, in the opposite direction. 
There are three hurricane regions ; the West Indies, the Indian Ocean, and 
the China Sea. In the last they are called Typhoons. 

1023. Tornadoes. — Tornadoes, or Whirlwinds, are as 
violent as hurricanes, but more limited in. extent. They 
are rarely more than a few hundred yards in breadth and 
twenty-five miles in length. Though lasting but a few sec> 
onds in a given place, they are frequently most disastrous 
in their effects, prostrating forests, overturning buildings, 
and ravaging the whole face of the country. 

1024. Water-spouts. — A Water-spout is a phenomenon 
frequently observed at sea, consisting of a column of water 
raised sometimes to the height of a mile and tapering from 
each end towards the centre. It is supposed by some to 
be produced by a whirlwind of great intensity ; by others 
it is attributed to electrical influences. 

the effect of the simoom on the animal world ? When do land and sea breezes blow, 
and how are they produced ? 1021. What are Variable Winds ? Where do they 
chiefly prevail ? 1022. What are Hurricanes ? By what are they distinguished? In 
what direction do they revolve? Name the three hurricane regions. 1023. What 
are Tornadoes? Describe their effects. 1024. What is a Water-spout ? By what is 
It produced ? Give an account of the way in which it is formed. 1025. What doos 



404 METEOROLOGY. 

Water-spouts are formed as follows : — From a dark cloud a conical pillar 
is seen to descend with its point downward. As it approaches the water, the 
latter becomes violently agitated, and a similar column rises from it, point 
upward. The two finally unite, forming a continuous column from the cloud 
to the water. After remaining joined for a time, they again separate into 
two columns, one of which is drawn up into the cloud, while the other pours 
down in the form of heavy rain. Sometimes the two columns are dispersed 
before a junction is effected. 

1025. Atmospheric Moisture. — The atmosphere al- 
ways contains more or less moisture, derived from the 
earth's surface, particularly those portions of it that are 
covered with water, by the process of evaporation. When 
the air contains as much moisture as it is capable of holding 
at any given temperature, it is said to be saturated. 

The higher the temperature of air, the more moisture it is capable of re- 
ceiving. At 32° F., it will hold only »/ 160 of its own weight of watery vapor ; 
while at 113° it will receive eight times as much, or V20 of its own weight. 

1026. The earth gives out incredible quantities of moisture by evaporation. 
Experiments prove that an acre of ground apparently parched by the sun 
sends forth into the air over 3,000 gallons of water in 24 hours. Of course 
much greater quantities are evaporated from a moist soil and from surfaces 
covered with water. 

1027. The Hygrometer. — The amount of moisture in the 
atmosphere is ascertained with an instrument called the 
Hygrometer. Hygrometers are made on different principles. 

In some, the degree of humidity is indicated by the elongation of a hair, 
a fibre of whalebone, or some other animal substance which readily absorbs 
moisture and is increased in length by so doing. In others, it is shown by 
the increase of weight in some substance that absorbs moisture, such as 
sponge, cotton, or potash. In the more delicate instruments, the degree of 
moisture is shown by the greater or less facility with which it is condensed 
from the air in the form of dew on a cold surface. The more moisture in the 
air, the less cold will be required to condense it into dew. 

1028. Fog — Clouds. — When the air is cooler than the 
earth, the moisture imparted to it in the manner just de- 
scribed is partially condensed and thus rendered visible, 
forming either fog or clouds. The only difference between 
the two is in their height. When the condensation takes 

the air always contain ? When is it said to be saturated f On what does the amount 
of moisture that air can receive depend ? 1026. How much moisture does the earth 
give out by evaporation ? 1027. What is the Hygrometer? Mention the different 
principles on which the hygrometer is made. 1028. When are Fog and Clouds 



CLOUDS. 405 

place near the earth's surface, fog is the result ; when in the 
upper regions of the atmosphere, clouds. 

1029. Kinds of Clouds. — Clouds are divided into dif- 
ferent classes, the principal of which are the Nimbus, the 
Cumulus, the Stratus, and the Cirrus. 

The Nimbus, or rain-cloud, is a dense mass of vapor, of a leaden gray or 
blackish color, with a lighter tint on its edges. — The Cumulus has the appear- 
ance of many dense whitish clouds piled up one on another ; or of a vast hem- 
isphere with its base on the horizon, and peak rising above peak, looking 
like huge hills of snow when illumined by the sun. The cumulus may be 
called the cloud of day, and is an indication of fair weather. — The Stratus 
consists of a number of horizontal layers of cloud, not very far removed from 
the earth's surface. Forming at sunset and disappearing at sunrise, it may 
be called the cloud of night. — The Cirrus (called cat's tail by sailors) is a 
fleecy cloud, composed of thin feathery filaments disposed in every variety 
of form. The cirrus is the highest of all clouds, frequently reaching an alti- 
tude of from three to five miles. It is no doubt often composed of snow- 
flakes, as the temperature of the regions in which it floats must be cold 
enough to freeze the watery particles. 

1030. Dew. — When the moisture of the atmosphere 
comes in contact with an object colder than itself, it is con- 
densed and deposited on the surface. This is the way in 
which Dew is formed. 

A glass of ice-water on a warm day is almost immediately covered with a 
fine dew. So, in winter, when a number of persons are in a warm room, the 
moisture imparted to the air by their breath is condensed on the window- 
panes by the cold air without, and then sometimes frozen, giving them a 
beautiful frosted appearance. — Just so, in the evening, when objects on the 
earth's surface are cooled down by radiation, the moisture of the atmosphere 
is deposited on them in the form of dew. 

1031. Dew is never abundant except during calm serene nights. It is 
generally more plentiful in spring and autumn than in summer, because the 
difference between the temperature of day and night is greater in those sea- 
sons. The quantity of dew precipitated on different bodies depends much 
upon their nature. Thus grass and leaves will frequently be found glistening 
with crystal drops at sunrise, when gravelled walks, stones, wood-work, and 
metallic surfaces, are comparatively dry — another striking proof of the wis- 
dom with which Providence, orders the economy of nature. 

1032. Frost is nothing more than frozen dew. 

formed ? What is the difference between them ? 1029. Name the different kinds of 
clouds. Describe the Nimbus. The Cumulus. The Stratus. The Cirrus. 1030. Un- 
der what circumstances is Dew formed ? What familiar instances of the formation 
of dew are mentioned? 1031. When is dew most abundant? How does its orecipi- 



406 METEOROLOGY. 

1033. Rain. — Rain is water taken up by the air in the 
form of vapor and returned to the earth in drops. 

When two masses of damp air differing considerably in temperature are 
mingled, they become incapable of retaining the same amount of moisture 
which they held while they remained apart. The excess is precipitated in 
the form of rain, the vesicles of vapor under the influence of mutual attrac- 
tion blending together and forming drops. 

Some parts of the earth never have any rain, vegetation, when it exists 
at all, being supported entirely by dew. This is the case with Peru, the 
Desert of Sahara, portions of Arabia and Egypt, and extensive districts in 
Central Asia. In other parts, for example Guiana, it rains almost con- 
stantly. The Island of Chiloe has a rather moist climate ; the people there 
have a current saying, that it rains six days in the week and is cloudy the 
seventh. 

1034. Snow. — Snow consists of the watery particles of 
the atmosphere frozen for the most part in a crystalline form. 

Viewed through a microscope, snow-flakes exhibit forms of great beauty 
and endless variety. Between six and seven hundred different forms have been 
distinguished, many of them belonging to the six-sided system of crystals. 

Snow of a beautiful crimson color and a delicate green has been found in 
different parts of the world. These tints are due to minute plants or animal- 
cules in different stages of development. 

1035. Hail. — Hail consists of globules of ice formed in 
the atmosphere by the congelation of its moisture and pre- 
cipitated to the earth. 

Hail is produced by an intense degree of cold in the atmosphere, and is 
generally accompanied with electrical phenomena. It is rare at the level of 
the sea within the tropics, and in high latitudes is totally unknown, being 
most abundant in temperate climates. Hail-storms seldom continue a quarter 
of an hour, but while they last large quantities of ice fall. The stones are 
generally pear-shaped, and frequently weigh ten or twelve ounces. Masses 
weighing 6, 8, and even 14 pounds, have been known to fall. 

tation show the goodness of Providence ? 1032. What is Frost ? 1083. What is Rain ? 
How is rain formed ? What parts of the earth never have any rain ? Where does 
it rain almost constantly? 1034. Of what does Snow consist? What is the form of 
snow-flakes ? Of what color has snow sometimes been found ? How is this account- 
ed for ? 1035. Of what does Hail consist ? How is it produced ? Where is it most 
frequent ? What is the shape of hail -stones ? How -large have they been known 
to fall? 



FIGURES 



For the convenience of the pupil during recitation, the Figures 
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B >»H 



Fig. 253. 





FIGURES. 



431 




Fig. 256. 



Fig. 258. 









432 



NATURAL PHILOSOPHY. 




FY/-'" 



Fig. 260. 




Fig. 261. 




Fig. 262. 




FIGURES. 



433 





Fig. 288. 







434 



NATURAL PHILOSOPHY. 



Fig. 269. 




Fig. 274. 



m 



i ^ 



Fig. 285. 



A 



Fig. 286. 



Fig. 287. 





435 



Fig. 299. 




Fig. 300. 




Fig. 302. 



Fig. 29T. 




Fig. 301. 
b b b b 



vww 

a a a a a 
b b b 

uuruu 



a a a 



Fig. 



R 




At 



J* 




436 



NATURAL PHILOSOPHY. 



Fig 316. 



Fig 311 




Jl q 




Fig. 319. 



Fig. 318. 





FIGURES. 



457 



Fig. 323. 





Fig. 326. 




4J8 



NATURAL PHILOSOPHY. 

Fig. 828. 




439 




Fig. 336. 




HOLTZ'S ELECTRICAL MACHINE 



INDEX. 



[the figures eefee to pages, not to sections. 



A. 

Aberration, chromatic, 259. 

Acoustics, defined, 274. 

Actinism, 257. 

Action, defined. 42. Equal to reaction, 43. 

Adhesion, defined, 21. Experiments il- 
lustrative of, 21. 

Adjutage, different forms of, 155. 

Aeriform bodies, defined, 8. 

Affinity, chemical, 9, 195. 

Agents, defined, 7. 

Air, composition of, 9. Tends to stop 
motion, 36. Eesistance of the, 54; ef- 
fect of, 59, 62. Exists in every sub- 
stance, 166. Is impenetrable, 166. Is 
compressible, 167. Is elastic, 167. Has 
weight, 168. Density of, at different 
levels, 174. Effects of its rarity, 175. 
Rarefied by heat, 175. Navigation of 
the, 177. Essential to life and combus- 
tion, 184. Supports a column of water 
from 32 to 34 feet high, 187. A non- 
conductor of heat, 201. 

Air-gun, the, 168. 

Air-pump, the, 177. Single-barrelled, 
178. Double-barrelled, 179. Experi- 
ments with, 180. 

Alfred the Great, his mode of measuring 
time, 126. 

Amazon, the, its fall, 156. Its discharge, 
156. 

Anemometer, the, 401. 

Angle, defined, 35. Vertex of, 35. Eight, 
35. Obtuse, 35. Acute, 35. Visual, 264. 



Anode, defined, 324. 

Aphelion, of a planet, 374. 

Apogee, 389. 

Apple-cutter, the, 180. 

Aqueducts, of the ancient Eomans, 133. 

Arc, defined, 34. 

Arch, the voltaic, 328. Celestial, 384. 

Archimedes, reasoned by induction, 10. 
Explained the properties of the lever, 
94. Discovered the leading principles 
of specific gravity, 146. His screw, 162. 
Fired the Roman fleet with concave mir. 
rors, 194. 

Aristotle, his doctrine respecting falling 
bodies, 54. 

Armature, of magnets, 335. 

Ascending bodies, 59. Height reached 
by, 00. 

Asteroids, the, 392. 

Astronomy, defined, 368. Fundamental 
facts of, 369. 

Athermanous substances, defined, 206. 

Atmosphere, the, 166. Pressure of, 169, 
174 ; mode of transmitting mails by the, 
182. How heated by the sun, 204, 
Moisture of, 404. 

Atomic theory, 17. 

Atoms, what they are, 17. 

Attraction, of gravitation, 20. Molecular, 
21. Capillary, 146. Between floating 
bodies, 149. Electrical, 289, 290. Mag- 
netic, 337. 

AtwoocPs machine, 56, 57. 

Aurora borealis, 303, 312. 

Axis, of a sphere, 71. 



INDEX. 



441 



B. 

Bacon, Roger, invented spectacles, 263. 

Balance, the, 96. Of a watch, 128. 

Ballistic pendulum,, the, 64. 

Balloons, why they rise, 53. Eose, why 
they lose their buoyancy, 150. Inven- 
tion of, 176. 

Barometer, the, 171. The wheel, 172. Its 
use as a weather-guide, 172. 

Barometer gauge, the, 180. 

Base, of a body, 72. 

Battery, the electrical, 300. The galvanic, 
819 ; the trough, 320; Smee's, 320; Dan- 
iell's, 321; Grove's, 322; Bunsen's, 322; 
theory of, 323. The thermo-electric, 332. 
The relay, 361. 

Bay of Fundy, tides of the, 157. 

Beam, a, of light, 230. 

Bell, vacuum, 183. Electrical, 301. 

Bellows, hydrostatic, 137. Principle of the 
common, 170. 

Bladder-glass, the, 181. 

Blindness, cause of, 263. 

Boats, propulsion of, 160. Shape of, 161. 

Bode, his law, 376. 

Body, a, what it is, 7. A simple, 8. A 
compound, 8. The simple bodies, 8. 
When it stands and falls, 73. 

Boilers, how made, 225. 

Boiling, process of, explained, 203. 

Bottle, thousand-grain, 141. 

Bottle imps, 167, 182. 

Breadth, defined, 12. 

Breatlvmg, process of, explained, 170. 

Breezes, land and sea, 402. 

Brewster, Sir David, invented the kalei- 
doscope, 241. 

British Channel, tides of the, 157. 

Brittleness, defined, 23. 

Buckets, of a wheel, 158. 

Buffon, his experiment with concave mir- 
rors, 194. 

Burning, process of, 195. 
isses, 243, 251. 



Calms, region of, 402. 

Camera obscura, 266. Draughtsman's, 
267. Daguerreotypist's and Photograph- 
er's, 267. 



Canals, principle of their construction, 
134. 

Cancer, tropic of, 388. 

Capillary Attraction, what it is, 146. 
Cause of, 146. Familiar examples of, 
147. Laws of, 148. Interesting facta 
connected with, 149. 

Capricorn, tropic of, 388. 

Capstan, the, described, 105. 

Carbon, combines with oxygen to pro- 
duce animal heat, 196. 

Carbonic acid, found at the bottom of 
wells, 140. 

Cathode, defined, 324. 

Catoptrics, 236. 

Centre of gravity, 70. Of magnitude, 71. 
Of motion, 71. 

Centre of gravity, what it is, 70. How to 
find it, 71. In man, 77. Tends to get 
as low as possible, 78. 

Centrifugal Force, defined, 37. Exam- 
ples of, 38, 39. Law of the, 39. Its ef- 
fect on revolving bodies, 39. Apparatus 
to illustrate the, 40. Gave its form to 
the earth, 41. 

Centripetal Force, defined, 37. 

Ceres, when discovered, 375. 

Chemical action, a source of heat, 195. 

Chemical affinity, 9, 195. 

Chemistry, defined, 9. 

Chinese, the, early acquainted with gun- 
powder, 63. First used the magnet in 
navigation, 342. 

Chord, what it is, 285. 

Chords, vocal, 286. 

Chromatics, 254. 

Chronometers, how accurate, 127. 

Circle, defined, 34. Simple galvanic, 319. 
The arctic, 389. The antarctic, 389. 

Circumference, defined, 34. How divided, 
35. 

Cirrus, the, defined, 405. 

Clepsydra, the, 126. Described, 153. In- 
vented by Ctesibius, 187. 

Clocks, how regulated, 68. History of, 
126. Pendulum applied to, 126. "Works 
of, 127. Electro-magnetic, 364. 

Clouds, how formed, 404. Kinds of, 405. 

Coat, choroid, 262. Sclerotic, 262. 

Cogs, what they are, 123. 

Cohesion, defined, 21. 

Cold, what it is, 192. 



442 



INDEX. 



Colors, the primary, 255. Difference of; 
explained, 256. Complementary, 257. 

Columbus, his discovery respecting the 
variation of the compass, 343. Saved 
himself and his men by predicting an 
eclipse, 396. 

Combustion, what it is, 195. Produces 
most of our artificial light, 231. 

Comets, what they consist of, 39T. Their 
orbits, 397. Their velocity, 397. Their 
number, 398. 

Compass, land or surveyor's, 341. Mari- 
ner's, 342. Boxing the, 342. 

Compressibility, 19. Of air, 20. 

Concord, 285. 

Condensation, 212. Of steam, 218. 

Condenser, the, 184. Of the steam-en- 
gine, 222. 

Conductometer, the, 199. 

Conductors, of heat, 199. Of electricity, 
293. 

Conjunction, 379. 

Constellations, 386, 399. 

Convection, of heat, 202. 

Copernicus, revived the true theory of the 
universe, 370. 

Cornea, the, 261. Use of, 262. 

Couronne des tasses, the, 320. 

Crank, the, 124. 

Crown-xcheel and pinion. 123; combined 
with a trundle, 123. 

Ctesibius, invented the lifting-pump. 186. 
Invented the clepsydra, 1S7. Supposed 
to have invented the water-organ. 284. 

Cumulus, the, described, 405. 

Cup, Tantalus's, 1S6. The phosphorus, 305. 

Cupping-glasses, principle of, 175. 

Curb, of a watch. 129. 

Curves, magnetic, 338. 

Cylinder, defined. 36. 



Daguerreotype process, the, 268. 

Dams, should increase in strength at the 
base, 136. 

Dead-point, the. of a crank. 125. 

Density, 19. In optics, 246. 

Descartes, advanced the undulatory the- 
ory of light, 229. 

Dew, how formed. 405. 

Diagonal, denned. 35. 

Diamagnetism, 367. 



Diameter, defined, 34. 

Diathermanous substances, defined, 200. 

Dionysius, ear of, 281. 

Dioptrics, 246. 

Dip, magnetic, 340. 

Direction, line of, 71. 

Discharger, the jointed, 298. The uni- 
versal, 298. 

Discord, 285. 

Dispersion, of light, 259. 

Distillation, process of, described, 212. 

Diving-bell, the, 166. 

Divisibility, defined, 17. Instances of. IS 

Double cone, may be made to roll up an 
inclined plane, 80. 

Draft, how produced in a chimney, 176. 

Driver, the, 120. 

Drum, the, 2S3. 

Ductility, defined, 26. Of platinum, 26. 
Of gold, 26. Of glass, 26. 

Du Fay, his theory respecting electricity, 
291. 

E. 

Ear, the human, 287. 

Earth, the, owes its form to the centrifu- 
gal force, 41. Magnetic poles of, 344. 
Form of, 382. Motions of, 383. Orbit 
of, 384. Phases of, to the moon, 390. 
How it would look from the fixed stars- 
395. 

Ear-trumpet, the, 280. 

Echoes, 279. 

Eclipse, of the sun, how produced, 395. 
Annular, 396. Of the moon. 396. 

Ecliptic, the, 385. Obliquity of, 385. 

Eel, the Surinam, 316. 

Elasticity, defined, 24. Perfect, 24. Be- 
longs to hard solids, 24. Of steel, 24. 
A limit to, 25. 

Electricity, a source of fight, 232. "What 
it is, 289. Nature of, 290. Sources of. 
290. Developed by friction, 291. Vitre 
ous, or positive, 292. Kesinous, or nega 
tive, 292. Conduction of, 293. Path of. 
294. Velocity of; 294. Machines for de 
veloping, 294; experiments with, 301 
Mechanical effects of the passage of, 304 
From steam, 310. Atmospheric, 310 
Voltaic, 316. Difference between fine 
tional and voltaic, 324. Developed by 
heat, 332. Connection between mag 



INDEX. 



443 



netism and, 352. Developed by mag- 
netism, 366. 

Electric Light, 328. Edison's, 455. 

Electrodes, -what they are, 323. 

Electro-magnetism, defined, 349. As a 
motive power, 357. 

Electro-magnets, 356, 357. 

Electro-metallurgy, 326. 

Electrometer, the, 309. The quadrant, 
309. 

Electrophorus, the, 308. 

Electroscope, the, 308. 

Electrotyping, process of, described, 327. 

Elements, sixty-four in number, 8. Di- 
vided into metals and non-metallic, 8. 
The non-metallic enumerated, 9. 

Endless band, 121. 

Endosmose, 150. 

Engine, defined, 88. Atmospheric, 219. 
Steam, 219. Hero's, 219. Marquis of 
"Worcester's, 220. Savery's, 221. New- 
comen's, 222. Watt's, 222. The low 
pressure, 226. The high pressure, 226. 
The locomotive, 226. 

Equator, the magnetic, 341. The celes- 
tial, 385. 

Equilibrium, stable and unstable, 79. 

Equinoctial, the, 385. 

Equinoxes, 385. Precession of the, 386. 

Escapement, of clocks, 127. Of watches, 
128. 

Esquimaux, why they thrive on fat, 196. 

Evaporation, 211. 

Exosmose, 150. 

Expansibility, 19. Of air, 20. 

Expansion, 207. 

Experiment, what it consists in, 10. 

Extension, defined, 12. 

Eye, the, 260. Parts of, 261. Adaptation 
of, 265. 



Faculce, 372. 

Fahrenheit, his thermometrical scale, 214. 

Falling bodies, velocity of, 54. Law of, 

56. Kules relating to, 58. 
Faraday, his theory of electricity, 292. 
Fata morgana, 248. 
Figure, defined, 12. 
Fire, St. Elmo's, 311. 
Fire-alarm, electro-magnetic, 365. 
Fire-balls, 312. 



Fire-engine, principle of the, 188. 

Fire-escape, the, 107. 

Fire-house, the electrical, 307. 

Fish, how they rise and sink in water, 145. 
Electrical, 316. 

Flame, how produced, 195. 

Float-boards, 158. 

Fluids, embrace liquids and aeriform bod- 
ies, 8. Difference between them and 
solids, 8. Non-elastic, 25. Elastic, 25. 
Division of elastic, 165. 

Flute, the, principle of, 283. 

Flyer, the electrical, 305. 

Fly-wheel, the, 125. 

Focus, the principal, 242. The virtual, 244. 

Fog, how formed, 404. 

Force, defined, 7, 26. Striking, 31. Cen- 
trifugal, 37. Centripetal, 37. Correlation 
and conservation of forces, 193. 

Forge-hammer, the, 124. 

Fountains, how high they rise, 132. Vac- 
uum, 181. 

Franklin, his theory respecting electric- 
ity, 292. Proved lightning to be produced 
by an electric discharge, 312. Invented 
the lightning-rod, 315. 

Friction, what it is, 37, 85. How it op- 
poses motion, 85. Kinds of, 85. Slid- 
ing, converted into rolling, 86. Laws 
of, 86, 87. Modes of lessening, 87. Uses 
of, 88. Of one wheel on another, 120. 
Of water against the sides of pipes, 155. 
Of a stream against its banks, 156. En- 
ables the wind to produce waves, 156. A 
source of heat, 197. A source of electric- 
ity, 290. 

Frost, what it is, 406. 

Fulcrum, what it is, 94. 

Furnace, of a steam-engine, 226. 

Fusee, of a watch, 128. 

Q. 

Galaxy, the, 400-. 

Galileo, his doctrine respecting falling 
bodies, 54. Invented the pendulum, 67. 
First made a practical use of the tele- 
scope, 272. Established the truth of the 
Copernican system, 371. 

Galle, Dr., discovered Neptune, 394. 

Galleries, whispering, 281. 

GaVoani, discovered voltaic electricity, 31T. 
His experiment, 317. 



444 



INDEX. 



Galvanism, 316. {For particulars, see 
Voltaic electricity.) 

Galvanometer, the, 351. With astatic 
needle, 352. 

Gamut, the, 284. 

Gases, what they are, 165. Specific grav- 
ity of, how found, 143. Exhibit endos- 
mose and exosmose, 150. Conducting 
power of, 201. Expansion of, 210. 

Gearing, what it is, 121. 

Gioia, Flavio, improved the compass, 342. 

Glottis, the, 286. 

Governor, the, 225. 

Gravitation, defined, 20. Circumstances 
attending its discovery, 47. Facts es- 
tablished respecting it, 47. Direction 
of, 48. Laws of, 49. 

Gravity, terrestrial, 46. Laws for the 
force of, 49. Sometimes causes bodies 
to rise, 53. Centre of, 70. Used as a 
motive power, 81. Specific, 139. Ta- 
bles of specific, 144. 

Guericke, invented the air-pump, 177. 
His famous experiment, 178. First con- 
trived an electrical machine, 295. 



Gunpowder, principle on which it acts, 
63. Invention of, 63. Too large charges 
dangerous, 65. Apparatus for firing, 
with electricity, 308. 



Hail, its disastrous effects, 59. How 
formed, 406. 

flair-spring, the, of a watch, 128. 

Halos, what they are, 260. 

Hand-glass, the, 180. 

Hardness, defined, 22. "Wanting in flu- 
ids, 22. Of various solids compared, 22. 

Harmony, what it is, 285. 

Harp, ^Eolian, 283. 

Heat, what it is, 192. Dynamic theory. 
192. Sources of, 193. The sun's, 193; 
how accounted for, 193 ; how it may be 
increased, 194 ; how far it penetrates 
into the earth, 194. Below the earth's 
surface, 194. Produced by chemical 
action, 195. Animal or vital, 196. Pro- 
duced by mechanical action, 197. From 
friction, 197. From percussion, 197. 
Mechanical equivalent of, 198. Produced 
by electricity. 198. Diffusion of, 198 ; by 



conduction, 199 « by convection, 20? ; by 
radiation, 203. Kadiant, 204; law oi, 
204; reflection o^ 205; absorption of 
206; transmission of, 206. Kffects of. 
207. Instruments for measuring*, 213. 
Specific, 216. 

Helix, the, 355. Masnetizing power of, 
355. Itself a magnet, 365. 

Hemispheres, the Magdeburg. 178. 

Hero, his steam-engine, 213. 

Herschel, his telescope, 273. Discovered 
Uranus, 375. 

Hiero, golden crown of, 145- 

Hooke, Dr., added the hair-spring to the 
balance, 127. 

Horizon, the sensible, 384. The rational, 
384. Poles of the, 385. 

Horse, the, strength of, 82. 

Horse-power, defined, 84. 

Humor, aqueous, 261. Vitreous, 261. 

Hurricanes, 403. 

Huyghens, applied the pendulum to clock- 
work, 67. Unfolded the undulatory 
theory of light, 229. 

Hydratdics, defined, 152. 

HydravMcon, the, 284. 

Hydrogen, the lightest substance knowr» 
144. Used for inflating balloons, 176 
Produces musical sounds, 284. 

Hydrometer, the, 142. 

Hydrostatics, defined, 130. Law of, 131. 
Hydrostatic paradox, 137. Hydrostatic 
bellows, 137. Hydrostatic press, 138. 

Hygrometer, the, 404. 



Ice, process of its formation, 210. 
Iceland spar, exhibits double refraction, 

252. 
Image, an, what it is, 239. 
Impenetrability, defined, 13. Of air, 13. 

Instances of, 13. 
Incandescence, 213. 
Incidence, angle of, 46. Equal to angle of 

reflection, 46. 
Inclined Plane, the, 110. Law of, 110. 

Practical applications of, 111. Law of 

bodies rolling down. 111. 
Indestructibility, defined, 13. Instances 

of, 13. Anecdote illustrative of 13. 
Induction, electrical, 309. Magnetic, 346. 



INDEX. 



445 



Inertia, defined, 15. Examples of, 15. 
Experiments illustrative of, 15, 16. Pro- 
portioned to a body's weight, 17. 

Instruments, optical, 266. Stringed, 282. 
Wind, 283. 

Insulators, 294. 

Iridium, one of the hardest metals, 22. 
The heaviest known substance, 144. 

Iris, the, 261. Use of, 262. 

VOM, great tenacity of, 23. 

J. 

Jar, the Leyden, 299. 
Juno, when discovered, 375. 
Jupiter, velocity of light ascertained from 
the eclipses of one of its moons, 234. Its 
59. Details of the planet, 392. 






K. 

Kaleidoscope, the, 241. 
Kepler, his laws, 377. 

I*. 

Lamp, principle on which it burns, 147. 

Landes, shepherds of, 78. 

Lantern, a species of wheel, 123. Used 
by King Alfred, 126. The magic, 271. 
Phantasmagoria, 271. 

Larynx, the, 286. 

Lava, discharge of, accounted for, 195. 

Leaves, what they are, 122. 

Length, defined, 12. 

Lens, the crystalline, 261. 

Lenses, what they are, 246. Classes of, 
249. Refraction by convex, 250. Ee- 
fractioD by concave, 251. Achromatic, 
259. 

Le Sage, first attempted to transmit mes- 
sages by electricity, 263. 

Level, the spirit, 134. The water, 135. 

Lever, what it is. 94. Of the first kind, 
94, 95. Practical applications of the, 
98. 100, 102. The bent. 99. The com- 
pound, 99. Of the second kind, 99. Of 
the third kind. 101. Perpetual, 104. 
Often combined with the screw. 115. 

Le Terrier, his prediction verified by the 
liscovery of Neptune, 394. 

Life-boats, principle of, 144. 

Life-preservers, principle of. 144. 

Light, what it is, 229. Corpuscular the- 
ory of, 229. Undulatory theory of, 229. 



! Sources of, 231. Of the sun, 232. Of 
the stars, 232. Propagation of, 232. Ve- 
locity of, 233. Intensity of, at different 
distances, 234. Reflection of, 236. Re- 
fraction of, 245. Laws of refracted, 246. 
Polarization of, 253. Dispersion of, 259. 
The electric, 328. The zodiacal, 373. 

Lightning, 312. Effects of, 313. 

Lightning-rod, the, 313. 

Lights, northern, 303. 

Line, a right, defined, 34. Parallel lines 
defined, 34. A curve, 34. The neutral, 
of a magnet, 334. 

Liquefaction, 210. 

Liquids, defined, 8. How they differ from 
solids, 130. Have little cohesion, 131. 
Compressibility of, 131. Not devoid of 
elasticity, 131. Pressure of, 135. Eule 
for finding then pressure on the bottom 
of a vessel, 137. Specific gravity of, 141. 
Exhibit endosmose and exosmose, 150. 
Flow of, through orifices, 152. Flow 
of, in pipes, 155. Conducting power of, 
201. Expansion of, 209. Converted 
into vapor by heat, 211. Good conduct- 
ors of sound, 276. 

Living Force, 31. 

Loadstone, described, 334. 

Lock, on a canal, 134. 

Locomotive, the, 226. 

Lubricants. ST. 

Lungs-glass, the, 181. 

HI. 

Machines, what they are, 88. Can no* 
create power, 88. Law of, 89. Advan- 
tages of using, 90. All, combinations of 
the six mechanical powers, 120. Must 
be regular in their motion, 125. For 
raising water. 161. Electrical, 294. Cyl- 
inder, 295, 296. Plate, 297. Hydro- 
electric. 310. Magneto-electric, 367. 

Magic lantern, the, 271. 

Magnetism, defined, 333. Theory of, 343. 
Terrestrial, 344; intensity of, 345. By 
induction, 346. By the sun's rays, 347. 
By contact with a magnet. 347. De- 
veloped by electricity, 349. Connection 
between electricity and, 352. 

Magneto-electricity, 366. Medical use of, 
367. 

Magnets, what they are. 333. Natural 



446 



INDEX. 



334. Poles of, 334, 337. Power of nat- 
ural, 335. Armed, 335. Artificial, 335. 
Bar, 336. Horse-shoe, 336. Compound, 
336. Power of, how increased and di- 
minished, 337. Attraction of, 337; law 
of, 338. Polarity of, 338. Production 
of artificial, 345. 

Magnifying glasses, 251. 

Main-spring, the, of a watch, 128. 

Malleability, defined, 25. Of the metals, 
25, 26. 

Mariotte's law, 168. 

Mara, details of the planet, 391. 

Matter, defined, 7. Three forms of, 7. 
Universal properties of, 12. Accessory 
properties of, 21. 

Mechanical powers, the, 94. 

Mechanics, defined, 26. 

Medium, a, what it is, 231. A uniform, 
231. A dense, 246. A rare, 246. 

Melody, what it is, 285. 

Meniscus, what it is, 249. 

Mercury, details of the planet, 381. 

Meridian, the magnetic, 340. 

Metals, the principal, 9. Specific gravity of 
various, 144. Precious, how tested, 145. 
Protection of, by voltaic electricity, 328. 

Meteorology, defined, 401. 

Metius, supposed to have invented the 
telescope, 272. 

Microscope, wonders revealed by the, 18, 
270. What it is, 268. The simple, 268. 
The compound, 269. Solar, 270. Oxy- 
hydrogen, 270. 

Milky way, the, 400. 

Mill, Barker's, 161. 

Mill-stones, how made in France, 148. 

Mirage, 248. 

Mirrors, concave, Boman fleet fired with, 
194. What they are, 237. Plane. 238. 
Concave, 238. Convex, 238. Eeflection 
from plane, 240. Images formed by 
plane, 241. Eeflection from concave, 
242. Eeflection from convex, 244. 

Mississippi, the, its discharge, 156. 

Mixtures, freezing, 211. 

Mobility, defined, 20. 

Momentum, what it is, 29. Eule for find- 
ing the, 30. 

Monsoons, 402. 

Montgolfier brothers, balloons invented by 
the, 176. 



Moon, the, 389. Produces tides, 157. Size 

of, 3S9. Motions of, 389. Phases of, 390. 

New, 390. Gibbous, 390. Full, 390. 

How it appears through the telescope, 

391. Eclipses of, 396. 
Moons, of Jupiter, 392. Of Saturn, 393. 

Of Uranus, 393. 
Morse, his telegraph, 358. His telegraphic 

alphabet, 361. 
Motion, what it is, 27. Absolute, 27. Eel- 

ative, 27. Kinds of, 28. Uniform, 28. 

Accelerated, 29. Eetarded, 29. Firs, 

law of, 36. Second law of, 41. Simple, 

41. Eesultant, 41. Parallelogram of, 

42. Third law of, 43. Eeflected, 45; 
law of, 46. Eotary, may keep a body 
from falling, 76. Perpetual, 89. Cir- 
cular, how converted into rectilinear, 
124. Alternate up-and-down, how pro- 
duced, 124. Eeal and apparent, of the 
planets, 380. 

Motive powers, 81. 
Midtiplying glass, the, 252. 
Muschenbroeck, his electric shook, 300. 

N. 

Nadit\ the, 385. 

Natural Philosophy, defined, 9. Modes 
of investigation in, 10. Branches of, 11. 

Nebulae, 400. 

Needles, magnetic, 336. Horizontal, 336. 
Dipping, 336, 341. Astatic, 339. How 
to magnetize, 347. Effects of electric 
currents on magnetic, 349. 

Neptune, when discovered, 375. First 
called Le Verrier, 375. Details of th« 
planet, 394. 

Nerve, optic, 261, 262. Acoustic, 288. 

Newcomen, his steam-engine, 222. 

Newton, discovered the law of gravitation, 
47. Held the corpuscular theory of light, 
229. 

Nimbus, the, described, 405. 

Non-conductors, of heat, 199. Of elec- 
tricity, 293. 

Non-electrics, 293. 

Non-lwminous bodies, defined, 230. 

North star, distance of the, 399. 



Observation, what it consists in, 10. 
Occultation. 380. 



INDEX. 



447 



Ocean, the surface of, spherical, 131. 
Pressure of, at great depths, 136. 

Octaves, what they are, 284. 

Oersted, discovered the phenomena of 
electro-magnetism, 349. 

Oil, how extracted from seeds, 112. 

Opaque bodies, defined, 231. 

Opera-glass, the, 273. 

Opposition, 379. 

Optics, defined, 229. 

Organ, the, 284. 

Orifices, velocity of streams flowing 
through, 153. Course of streams flow- 
ing through, 154. Volume discharged 
from, 154. 

Oxygen, promotes combustion, 195. Com- 
bines with carbon to produce animal 
heat, 196. 



Paddles, of a wheel, 160. 

Pallas, when discovered, 375. 

Pallets, of an escapement, 127, 129. 

Parachute, the, 55. 

Paradoxes, 80. Hydrostatic Paradox, 137. 

Parallax, 395. 

Parallelogram, defined, 35. Of motion, 42 

Pascal, constructed the first barometer. 
171. 

Pencil, a, of light, 230. A diverging, 230. 
a converging, 230. 

Pendulum, the, what it is, 65. Laws of 
its vibration, 65, 66. Application to 
clock-work, 67. Vibrates differently in 
different latitudes, 67. Effect of heat 
on its vibrations, 68. Compensation, 68. 
Gridiron, 68. Ballistic, 64. Its use in 
clock-work, 127. 

Penumbra, the, 235. 

Percussion, a source of heat, 197. A 
source of light, 282. 

Perigee, 889. 

Perihelion, of a planet, 374. 

Perspective, the magic, 242. 

Perturbations, 894. 

Phantasmagoria, 272. 

Philosophy, meaning of the term, 9. 

Phonograph, the, 281, 451. 

Photographic process, the, 268. 

Photosphere, of the Sun, 872. 

Physics, defined, 9. 

Pile, Volte's, 318. The dry, 322. 



Pinions, defined, 122. 

Pipes, flow of liquids in, 155. 

Pisa, tower of, 75. Scene of an interest- 
ing experiment, 54. 

Pistol, the electrical, 304. 

Planets, the, 373. Secondary, 374. Pri- 
mary, 374. Inferior, 374. Superior, 374. 
Orbits of, 374. Table of, 375. Aspects 
of, 378. Are they inhabited, 380. 

Plating, 326. 

Pneumatics, defined, 165. 

Points, the cardinal, 341. Of the com- 
pass, 342. 

Polarity, magnetic, 338. 

Polarisation, of light, 252. 

Poles, of a galvanic battery, 323. Of nat- 
ural magnets, 334. Of artificial mag- 
nets, 337. Magnetic, of the earth, 344. 
Of the horizon, 385. 

Pores, what they are, 18. 

Porosity, defined, 19. Of various sub- 
stances, 19. 

Powers, the mechanical, 94. 

Press, book-binder's, 115. Bramah's hy- 
drostatic (or hydraulic), 188. 

Pressure, of liquids, 135. Of the atmos- 
phere, 169. 

Prisms, refraction by, 248. Decompose 
light, 255. 

Projectile, a, what it is, 60. Forces by 
which it is acted on, 61. Path of, 61. 
Eandom of, 62. 

Propeller, the screw, 160. 

Properties, universal, 12. Accessory, 12. 

Ptolemy, his system of the universe, 370. 

Pulley, the, 106. The fixed, 106. The 
movable, 107. White's, 108. Much of 
its advantage lost by friction, 109. 

Pump, the chain, 162. The lifting, 186. 
The forcing, 187. The centrifugal, 189. 
The stomach, 190. 

Pupil, the, 261. Of beasts of prey, 262. 

Pyramids, the most stable of figures, 74. 
Egyptian, 74. 

Pyrometer, the, 215. 

Pyronomics, defined, 192. 

Pythagoras, the first to use the term 
philosophy, 9'. Taught the true theory 
of the solar system, 370. 



Quadrant, defined. 35. 



448 



INDEX. 



Quadrature, 379. 
Quadrilateral, defined, 35. 
Quarter, first, of the moon, 390. Third, 
of the moon, 390. 

R. 

Race, a, what it is, 158. 

Rack and pinion, 124. 

Radiation, of heat, 204. 

Radius, defined, 34. 

Radius vector, the, 377. 

Rain, 406. 

Rainbow, the, 259. Primary and second- 
ary, 260. Lunar, 260. 

Ram, the hydraulic, 163. 

Random, 62. At what angle it is great- 
est, 63. 

Rarity, 19. In optics, 246. 

Rays, what they are, 230. Incident, 236. 

Reaction, defined, 43. Equal to action, 43. 
Examples of, 43. Often nullifies action, 
43. 

Reasoning, by induction, 10. By analo- 
gy, 10. 

Reaumur, his thermometrical scale, 214. 

Receivers, 111. 

Rectangle, defined, 36. 

Reflection, angle of, 46. Equal to angle 
of incidence, 46. Of light, 236; great 
law of, 238. 

Refraction, 245. Atmospheric, 247. By 
convex lenses, 250. By concave lenses, 
251. Double, 252. Its effect on the 
apparent position of the heavenly bod- 



s, defined, 210. 

Refrigerators, what their sides are filled 
with, 200. 

Regulator, of a watch, 129. 

Repulsion, between the particles of aeri- 
form bodies, 21. Between solids and 
liquids, 147. Electrical, 290. 

Resistance, what it is, 27. Appears in 
various forms, 84. 

Rest, what it is, 27. Absolute, 27. Eel- 
ative, 27. 

Restitution, force of. 24. 

Retina, the, 261, 262. Images formed on, 
264. 

Rhodium, one of the hardest metals, 22. 

Rivers, velocity of, how retarded, 156. 

Rocking horse, the, 76. 



Rocking stones, 79. 

Rocks, how rent, 136. 

Roemer, first used mercury in the ther- 
mometer, 214. Discovered the velocity 
of fight, 234. 

Rope-dancers, how they balance them- 
selves, 78. 

Rosse, Earl of his telescope, 273. 

Rotation, electro-magnetic, 353. 

Rubber, the, 289. 



Safes, what the sides are filled with, 200. 

Sap, how it ascends and descends in 
plants, 151. 

Saturn, details of the planet, 393. 

Savery, his steam-engine, 221. 

Scale, diatonic, 285. 

Scape-wheel, the, of a watch, 127. 

Screw, the, what it consists of, 114. Kinds 
of, 114. Advantage gained by, 114. 
Practical uses of, 116. Hunter's, 116. 
The endless, 117. Archimedes 1 , 162- 

Seasons, the change of, 386. 

Sea-water, heavier than fresh water, 144. 

Sea-saw, the electrical, 301. 

Selenite, polarizes light, 254. 

Self-luminous bodies, defined, 230. 

Shadows, 235. 

Shock, the electric, 299; anecdote con- 
nected with, 300. 

Shower, the mercury, 182. 

Signal-key, the, 360. 

Silurus electricus, the, 316. 

Simoom, the, 402. 

Siphon, the, 185. 

Sirius, its light compared with the sun's, 
232. Distance of, 399. 

Sirocco, the, 402. 

the principle on which it acts, 38. 
, why it rises, 176. 

Snow, protects vegetation, 202. How 
formed, 406. Colored, 406. 

Solids, defined, 7. Difference between 
them and fluids, 8. Specific gravity of, 
142. Porosity of, proved with the air- 
pump, 184. Expansion of, 207. Melted 
by heat, 210. 

Solstices, the, 388. 

Sonorous bodies, defined, 275. 

Sound, nature of, 274. Transmission of, 
275. Telocity of, 277. Distance to 



INDEX. 



449 



which it is transmitted, 278. Interfer- 
ence of, 279. Reflection of, 279. A mu- 
sical, how produced, 281; loudness of, 
281 ; pitch of, 281 ; quality of, 281. 

Spark, the electric, 297. Color of, 306. 
Length of, 307. Ignition by, 307. 

Speaking-trumpet, the, 279. 

Specific gravity, 139. Of liquids, 141. 
Tables of, 144. How to ascertain the 
weight of a body from its, 145. 

Spectacles, 263. 

Spectroscope, the, 372. 

Spectrum, the solar, 255. Properties of 
the, 257. Dark lines in the, 258. 

Speculum,, a, what it is, 237. 

Sphere, defined, 36. Axis of, 36, 71. Poles 
of, 36. Equator of, 36. 

Spheroid, oblate, 36. Prolate, 36. 

Spirit-level, the, 135. 

Spots, solar, 371. 

Springs, used as a motive power, 82. 

Springs, origin of, 133. Hot, 195. 

Square, defined, 36. 

Stability, of bodies, 72. Depends on the 
position ol the centre of gravity, 75. 
How increased, 76. Of a sphere, how 
increased, 79. 

Stammering, 287. 

Stars, the, a source of light, 232. Magni- 
tudes of, 398. Distance of, 399. Peri- 
odical, 399. Binary, 399. Telescopic, 399. 

Staves, of wheels, 123. 

Steani, the most effective of motive pow- 
ers, 83. Generation of, 216. Tempera- 
ture of, 217. Properties of, 217. Con- 
densation of, 218. Electricity from, 310. 

Steam-engine, Hero's, 219. De Garay's, 

220. Of De Caus, 220. Branca's, 220. 
Marquis of Worcester's, 220. Papin's, 

221. Savery's, 221. Newcomen's, 222. 
Watt's, 222. Parts of the, 223, 224. The 
low pressure, 226. The high pressure, 
226. The locomotive, 226; history of, 
227. 

Steel, elasticity of, 24. 
Steelyard, the, 97. 

Stephenson, improved the locomotive, 228. 
Stethoscope, the, principle of, 278. 
Stereoscope, the, 266. 
Still, the, described, 212. 
Stilts, used by French shepherds, 78. 
Stool, the insulating, 29S. 
29 



Stratus, the, defined, 405. 

Strength, of men and animals, used as a 

motive power, 82. Of materials, 91. Of 

rods and beams, 91. 
Striking Force, 31. Difference between 

it and momentum, 31. Rule for finding 

the, 31. 
Strings, of musical instruments, 282. 
Sucker, the, principle of, 170. 
Sun, the, a source of heat, 193. A source 

of light, 232. Size of, 371. Constitution 

of, 372. Motions of, 372. Eclipses of, 395. 
Sun-dial, the, 126. 
Syringe, the fire, 197. 
System, the Solar, 370. True theory of, 

taught by Pythagoras, 370; revived by 

Copernicus, 370. 



Tangent, defined, 34. 

Teeth, connect wheels, 122. 

Telegraph, electro-magnetic, 358. Morse's, 
358. House's, 362. Bain's, 362. The 
sub-marine, 362. The Atlantic, 363. 

Telephone, the, 366, 453. 

Telescope, the, 272. Refracting, 272. As- 
tronomical, 273. Terrestrial, 273. Re- 
flecting, 273. Herschel's, 273. Earl of 
Rosse's, 273. 

Temperature, what it is, 192. 

Tempering, how effected, 24. 

Tenacity, defined, 22. Distinguished from 
hardness, 22. Belongs to the metals, 22. 
Of different substances compared, 23. 
Of liquids, 23. 

Thermo-electricity, 332. 

Thermometer, the, 213. Invention ot, 
214. The differential, 214. 

Thickness, defined, 12. 

Thunder, 312. 

Thunder-house, the, 306. 

Tides, what they are, 157. How pro- 
duced, 157. Spring, 157. Neap, 157. 
Height of, 157. 

Tools, defined, 88. 

Top, why it does not fall when spinning, 
76. 

Tornadoes, 403. 

Torpedo, the, 316. 

Torricelli, proved the pressure of the at- 
mosphere, 171. 

Tourmaline, polarizes light, 254. 



450 



INDEX. 



Train, of wheels, 120. Of wheels and 
pinions, 122. 

Transit, of a planet, 380. 

Translucent bodies, defined, 231. 

Transparent bodies, defined, 231. 

Treadle, the, 125. 

Tremithick, constructed the first practi- 
cal locomotive, 227. 

Triangle, defined, 35. 

Tripoli, formed of fossilized animalcules, 
18. 

Tropics, the, 388. 

Trimdle, a, what it is, 123. 

Tubes, acoustic, 278. Aurora, 302. 

Turbine, the, 159. 

Tympanum, the, 288. 

Typhoons, 403. 

U. 

Uranus, when discovered, 375. Its for- 
mer names, 375. Details of the planet, 
393. 

V. 

Vacuum, what it is, 166. Torricellian, 
172. Fountain, 181. Bell, 183. 

Valoe, the safety, 226. 

Vaporization, 211. 

Vapors, what they are, 165. Conducting 
power of, 201. Expansion of, 210. 

Variation, magnetic, 340. Lines of no, 340. 

Velocity, what it is, 27. Kule for finding 
the, 28. Of various moving objects, 28. 

Ventriloquism, 287. 

Venus, details of the planet, 382. 

Verge, of a watch, 129. 

Vesta, when discovered, 375. 

Veta, the, 175. 

Views, dissolving, 272. 

Vision, 260. Defects of, 263. 

Voice, the human, 285; when said to 
change, 286. Of the inferior animals, 287. 

Volta, his theory respecting galvanism, 
318. His pile, 318. Invented the cou- 
ronne des tosses, 320. 

Voltaic electricity, 316. Effects of, 324. 
Decomposes, 325. Luminous effects of, 
328. Heating effects of, 329. Physio- 
logical effects of. 330. Medically applied, 
331. 

W. 

Watches, history of, 126. Works of, 128. 
How regulated, 128. Parts of. 129. 



Water, composition of, 9. Used as a mo- 
tive power, 82. Quantity of, on the 
earth's surface, 130. Finds its level, 131. 
Conveyed in pipes, 132. How conveyed 
by the ancient Eomans, 132. Its weight 
compared with air, 144. Wheels moved 
by, 158. Machines for raising, 161. Ex- 
pansion of, in freezing, 209. Decom- 
posed by the galvanic battery, 325. 

Water-clock, the, 126, 153. 

Water-organ, the, 284. 

Water-spouts, 403. 

Watt, his steam-engine, 222. 

Waves, how produced, 156. Height of, 157. 

Wedge, the, 112. Used for raising weights, 
112. Familiar applications of, 113. Ad- 
vantage gained by, 113. 

Weighing, double, 97. 

Weight, what it is, 50. Above and below 
the earth's surface, 50. Law of, 52. At 
different parts of the earth's surface, 53. 

Weight-lifter, the, 182. 

Wells, Artesian, 133. 

Wheel and axle, the, 103. Simply a re- 
volving lever, 103. Law of, 104. Dif- 
ferent forms of, 104. 

Wheels, friction, 88. Enter largely into 
machinery, 120. Modes of connecting, 
120. Different forms of the circumfer- 
ences of, 121. Toothed, 122. Varieties 
of toothed, 122. Spur, 122. Cog, 123. 
Mill, 123. Mortice, 123. Crown, 123. 
Bevel, 123. In watches, 129. Under- 
shot, 158. Overshot, 158. Breast, 159. 

Whirlwinds, 403. 

Width, defined, 12. 

Wind, used as a motive power, 82. How 
produced, 401. Velocity of, how meas- 
ured, 401. Constant winds, 402. Trade- 
winds, 402. Periodical winds, 402. Va- 
riable winds, 403. 

Windlass, the, described, 105. 

Wind-milU. 82. 

Worcester, his steam-engine, 220. 

Work, unit of, 84. 

Wrapping connector, 121. 



Zenith, the, 385. 

Zodiac, the, 386. Signs of, 886. 

Zodiacal Light, the, 373. 



APPENDIX 



THE PHONOGRAPH. 

The Phonograph (sound-recorder) is based on the 
principle that different sounds are produced by aerial vi- 
brations differing in amplitude and rapidity (§ 737). 

In the simple instrument devised by Edison (see Fig. 
337), sounds are permanently recorded in the material 
form of indentations made in tin-foil by an object vibrat- 
ing with the sound-waves ; and they are reproduced after 
any length of time by causing these indentations to com- 
municate the corresponding vibrations again to the air. 

Fig. 337- 




C is a cylinder, supported on an axle passing through the standards 
A, B. At one end of the axle is a crank, D, and at the other a fly-wheel, 
E. On the circumference of the cylinder is a narrow spiral groove. The 
axle is a screw, the threads of which are as far apart as those of the groove 
just mentioned, and it works in a nut at A so that the cylinder moves for- 
ward or backward according to the direction in which the crank is turned. 
F is an iron disk called the diaphragm, T b of an inch thick. This dia- 
phragm operates a small steel stylus, or point, placed just below its centre ; 
and it is so arranged that with the aid of the lever H G, turning on the pivot 



452 APPENDIX. 

I, it may be removed, or be brought over against the cylinder, and there 
fixed with the stylus working in the groove as the crank is turned. Over 
the thin iron disk to which the stylus is attached, and nearly touching it, 
is a thick vulcanite plate, with a hole in its centre, to which a funnel- 
shaped cavity extends from the upper surface. This forms a mouth- 
piece. 

Operation. — The cylinder having been first smoothly covered with tin- 
foil, and the diaphragm adjusted so that the stylus may press upon the foil 
over the groove, the operator speaks loudly and distinctly into the mouth- 
piece, at the same time turning the crank as uniformly as possible. The 
diaphragm is thus caused to vibrate in a manner exactly corresponding to 
the air- vibrations that produce the successive sounds. The cylinder moves 
regularly to the left, and the soft foil is marked with a series of indenta- 
tions varying in depth with the amplitude of the vibrations — that is, accord- 
ing to the modulations of the speaker's voice. In Fig. 337 the mouth-piece 
is toward us ; the stylus is on the other side of the diaphragm, opposite the 
hole in the centre ; the cylinder has moved nearly half its length ; and the 
permanent record of the sounds is seen in lines on the tin-foil. 

To reproduce the sounds, the diaphragm F is removed, and the cylin- 
der brought back to its starting-place by turning the crank in the opposite 
direction. The diaphragm is then replaced, and, a large tin or paper cone 
having been fixed in the vulcanite disk to reenforce the sound, the crank 
is turned as at first. The depressions and elevations of the foil are thus 
brought in turn under the stylus, and cause it, and the diaphragm along 
with it, to vibrate just as they did before. These vibrations produce simi- 
lar ones in the outer air; and thus, no matter what time has elapsed, the 
machine talks back what has been said to it, with the same inflections and 
quality of sound. With a good instrument, the reproduced sounds have 
been distinctly heard at a distance of 200 feet. 

To insure uniformity of motion, clock-work has been substituted for 
the crank ; and, for the sake of greater permanence, it has been suggested 
to stereotype or electrotype the indented tin-foil, or make the cylinder of 
some plastic material that will harden and retain the impressions. In an- 
other form of the instrument, the cylinder has been replaced with a circu- 
lar plate, having a spiral groove cut on both surfaces. 

The indented foil can readily be removed from the cylinder, sent as a 
phonograph letter to any point, and there, attached to a cylinder of the same 
size, will reproduce the recorded sounds at any time and as often as may 
be desired. Friends can thus communicate with each other in their own 
tones. The voices of the living can be perpetuated after death. The 
speeches of famous orators, the songs of distinguished vocalists, lectures, 
readings, dramas, in the exact accents of those who delivered them, can 
be multiplied indefinitely, ground out at pleasure by future generations, 
and enjoyed for hundreds of years. Such are a few of the results that the 
sanguine look for, when this great invention, still in its earliest infancy, 
shall have been matured. 



RECENT INVENTIONS. 453 



THE TELEPHONE. 



X X3Xff^ 



The Telephone is an instrument for immediately 
reproducing at a distance, by means of magneto-electric 
currents, various sounds, including those of the human 
voice. 

We found in § 911 that a steel bar is converted into 
a permanent magnet by winding a wire round it and 
passing through the wire a current of electricity from a 
galvanic battery. Allow such a permanent magnet to 
draw to itself a piece of soft iron, and, removing the 
wires from the bat- 
tery, join their ends lg " 
as in Fig. 338 ; then 
every time the iron 
a is withdrawn and 
returned — or, with- 
out actual contact, 
is brought very near 
to the magnet — an electric current is induced in the wire. 

If now the wires are extended and coiled round a 
second magnetic bar, this will be affected by the same 
magnetic disturbances as the first ; and thus, as the soft 
iron is played backward and forward at one end, each 
motion is felt as an electric pulsation at the other. 

This is the principle on which the Telephone works. 
The soft iron is represented by the diaphragm of the in- 
strument. The motions to and from the magnet are sim- 
ply the vibrations of the diaphragm, produced by sound- 
waves, and reproduced in a similar diaphragm, causing 
similar sound-waves, at the distant end of the wires. 

The telephone is of American origin. It is an application of principles 
long known, and several names are associated with the invention. The 
chief of these are Elisha Gray, of Chicago,— Prof. Bell, of the Boston Uni- 
versity, — Prof. Dolbear, of Tufts College, — and Thomas A. Edison. 

The principle of the telephone may perhaps be most easily understood 
by an examination of Dolbear' s simple form of the instrument, shown in 
Fig. 339. — N S is a permanent bar magnet, bearing on one end the helix H, 



454 



APPENDIX. 



of insulated copper wire. D is the diaphragm — a disk of sheet-iron V so of 
an inch thick, forming an armature to the magnet. T is the mouth-piece. 
A line of wire connects the transmitting instrument with a receiver, of pre- 
cisely the same construction, at some distant point. 



Fig. 339. 





Operation. — "When one talks or sings into the telephone, the mouth- 
piece T concentrates upon the elastic diaphragm D the sound-waves pro- 
duced by the voice and reflecting its loudness, pitch, and quality. Corre- 
sponding vibrations are produced in the diaphragm, the magnet is affected, 
and electrical currents are generated in the wire more or less strong accord- 
ing to the amplitude of the vibrations. The magnet at the other end of the 
wire is at once affected. Endued with an attraction by turns more and less 
intense, it causes the diaphragm near it to vibrate precisely like the other, 
and thus to generate sound-waves similar to those originally produced by 
the voice. The resultant sounds being subdued, the ear should be applied 
to the second telephone. With two telephones at each station, the opera- 
tors may hold one to the ear, the other to the mouth, and thus, though 
many miles apart, converse uninterruptedly. 

Various improvements have been made and are making in the instru- 
ment, to increase its loudness, distinctness, and effective working distance. 
Among these is the combination of two or more diaphragms and magnets, 
connected in the same circuit, with a single mouth-piece. Edison has re- 
cently obtained the best results by doing away with the vibrating dia- 
phragm, and substituting for it a rigid plate of metal which concentrate* 
the sound on a small carbon disk or button. 

The greatest distance at which the telephone has thus far been made to 
act, is 250 miles. Conversation has been carried on through the bodies of 
sixteen persons standing with joined hands, and through a submarine 
cable between operators in England and France. Two wires 'brming a 
single circuit wholly metallic possess some advantages ; but a si ngle wire 
connecting with the ground, which completes the circuit as in the case of 
the telegraph, may be used. When the telephone shall have been per- 
fected, it promises to some extent to supersede the telegraph, the wires of 
which can be utilized for its operation. 



BECENT INVENTIONS. 455 



EDISON'S ELECTRIC LIGHT. 

Edison's Electric Light is not the voltaic arch or arc 
(§ 846), but is produced by the incandescence of a spiral 
wire of platinum subjected to the action of an electric 
current (§ 847). It was long a problem how to maintain 
the platinum in a state of incandescence and yet prevent 
it from fusing. Mr. Edison has accomplished this by 
means of an automatic regulation of the current. 

In one form of Edison's invention, when the tempera- 
ture of the platinum spiral has nearly reached the melt- 
ing-point, a second wire, heated by the electric current 
or by radiation from the first, expands and acts on a lever 
in such a way as to complete a shorter circuit. The cur- 
rent, following this shorter circuit, is diverted for the 
time from the spiral, and the temperature of the latter is 
thus kept within the desired limit. As soon as the spiral 
loses some of its heat, the short circuit is broken, and the 
current resumes its original course. 

Mr. Edison has also patented an electric lamp, in which 
the heat generated in the incandescent spiral wire expands 
the air confined in an adjacent chamber. The pressure 
thus created pushes out a diaphragm bearing a platinum 
point, and brings this point in contact with a second, 
thereby completing a shorter circuit and diverting the 
current from the spiral. The temperature of the spiral 
and the air in the chamber then falls by radiation, the dia- 
phragm moves back, the platinum points separate, and 
the short circuit is broken — to be formed again just be- 
fore the melting-point of the spiral is again reached. 
These movements are so rapid as to be imperceptible, 
occasioning no diminution in the strength of the light. 

In this case and the former, the surplus current may 
be used to supply another lamp ; the surplus from this 
may be carried to a third, and so on till the energy is ex- 
hausted. The electric current may thus be subdivided 
so as to adapt the light to use in different apartments. 



%l\'^ 



* 



U8RA R y t 



